Good for you!
On this..... I used to think like that. I had preconceived ideas of what a "Bike Tour" was, what "kind" of person did one. And what one would be like. As things turned out, I was completely wrong on just about all counts.
The key to me finding all this out was a package deal along a river, staying in nice hotels and my gear being bussed from place to place. I got to see things in a completely different way, I got to feel a whole load of wonderful, positive things and I've never looked back since.
Maybe, just maybe your wife can still be seduced by the right offer. If yes, it's laying down a foundation for many, many travels near and far for as long as ye both can ride a bike.
(My bike adventures have evolved a bit since then, but without that initial exposure I doubt I'd ever have started).
No, honestly, my wife really wouldn’t enjoy that. The ideas of cycling on public roads, packing and unpacking every day, moving to multiple hotels etc all are just about everything she’d hate in a holiday. We know each other well, we have the things we enjoy individually and the things we enjoy together and we encourage each other to do all of those things. She takes trips for her stuff, I take them for mine and we have others we take together.
On a separate point, is she comfortable with you being away for 3 weeks in foreign parts? Some people need constant updates. No harm to have a chat about that in advance.
We’ve spent a fair chunk of our years in different places due to work. She’s just got back from 2 months working in New Zealand. Two years ago she spent 6 months working in Glasgow, a couple of years before that, I was away in Spain and Hungary working for almost 4 months. Our careers in the film industry have regularly seen us apart as well as working together.
I’m currrently sat in a friend’s house in Mexico where I’ve been enjoying diving, surfing and hiking with them on a break originally planned for 2020. I’ve regularly gone off into the hills with a backpack, and occasionally a dog too, and she knows I have the necessary skills to look after myself and the means to extricate myself or call for help if needed. We check in most evenings with each other and have well established connection needs and protocols as we’ve been doing this a long while. We’ll be absolutely fine being apart for three weeks
And
I'm thinking that the kind of cycling done for a triathlon and such a demanding bike club are not exactly the same kind of cycling you'll want to be doing in France or Spain or wherever? Or that might just be me.
Oh lord, I would not be doing triathlon type riding on a tour, totally different horses for courses

I enjoy a good road ride but I also enjoy the fun of just watching the world go gently by and taking my time to travel.
I'm not a great believer in being "fit" for a bike tour in the physical sense. There are no rules on how far I have to cycle to be a bike tourist, not how many meters I have to climb. If I'm not as fit as others I can happily ride shorter distances. Some of my best days on the bike have been less than 20 km.
I would much prefer to start a tour with some reasonable fitness. I totally get why some folks might go off and plan to get fit on a tour but being unfit at the start with a heavy bike I feel would just make it all so much harder. Totally agree on riding short distances on some days though, just look around and take time to enjoy where you are.
Feck most people. It's your tour. Ride it your way.
Sometimes booking trains can only be done maybe 3 months out. Looking in September won't work now. As Andy said, Maninseat61.
Ah, maybe it’s the timing then. Thanks, I should have thought of that. Yes, seat61 is someone I’ve followed for years, used info from there to plan a non-cycling) trip over in Europe last year.
You've talked about a couple of routes in France and one in Spain and here it's distance. I don't see anything about where you want to go or what you want to see?
I'd suggest you have a think about that. I always much prefer riding my bike in a place I want to be or on my way to a place I want to visit. It makes any unpleasantness more enjoyable.
We're all different. Some want to reach that peak. Others the sea. Some like to visit friends or family. What or where fires you up?
This will sound quite odd but honestly, i don’t really know what I want other than to spend some time cycling and camping, eating food and watching the world go by as I pedal. I mean, I could list a thousand places to go and explore but on this occasion I’m limited to three weeks max and I don’t want to spend much of it on the travel at beginning or end. If I travel via The channel it’s an extra two days on trains there and back from home, if I fly then I have to box everything at each end and park the car at the aiport. If I go via plymouth then it’s much easier because I can almost cycle straight from my front door but it limits me to western France or northern Spain, which is no bad thing for someone happy to live on ‘du pain, du vin, du Boursin’ etc
Not having toured by bike before, I figured somewhere mostly flat and with decent infrastructure would be useful to start and for me to see how I get on with the concept. Generally I prefer being in the mountains but that’s going to make for less relaxed touring or longer trips there and back but probably something I’ll do in the future. I enjoy the process of travelling slowly so just moving each day and getting used to a new method to do that is as much as I really need from this trip. The next adventure would be better planned after seeing how I get on with this one.
Remember, you'll be going solo (or so it seems now) for three weeks. Not everyone likes that. The reality of eating alone in busy places can take getting used to. No one to celebrate with on reaching that milestone or commiserating if things go pear shaped. Everything is that bit better if we're somewhere we want to be!
I love travelling with someone but also enjoy the freedom of travelling alone. Do I get lonely sometimes, yes absolutely, but I also enjoy going ‘oooh, lets spend this morning exploring some really interesting niche museum and then spend the afternoon tasting wines at a vineyard (my wife doesn’t drink so would be bored) without having to worry that the other person just wants to move on.
As suggested upthread a few times, a few get-away-for-the-weekend rides, packed as you expect to be will be very useful. Distance is probably less important. I've taken "the long way" to a not too far away campsite when I wanted to get away without too much time. I've camped up on a friend's farm when I wanted to test out a few things. There's a whole lot more to a camping bike tour than riding the bike.
Yes, I’m planning on a few of these. With camping, I’m pretty experienced and have my gear fairly well dialled. I’ve over 40 years of hiking and climbing behind me, there won’t be much change to what I’d need to pack on the bike compared to what goes in my backpack, apart from the folding chair that I’m definitely taking on the bike tour
Keep looking for inspiration. I think that' the most important.
CGOAB has a treasure trove of journals and a very useful
map feature. Personally, reading about a place as I cycle through it is a fabulous experience.
I often spend time browsing journals here and on CGOAB. In fact, a couple of years ago I spent some very enjoyable afternoons reading your own journal which I have to say was so well put together and very enjoyable. I got about as far as where the pandemic had stopped you but I haven’t caught up with what’s happened since then, I really should.
Now, to add to your destination confusion.....

One of the great places to explore for a beginner (and not so beginner) bike adventurer are the flatlands of the Netherlands. Excellent infrastructure. Interesting places. Great (and numerous) camping options. Not too many hills (but there can be wind) and, important for some - English speaking! Setting up camp in a small campsite the other campers will be all chat. Very friendly. Don't forget to pack a smile! Trains are easy too. Logistically difficult, perhaps, from Devon but lots of benefits.
I know the Netherlands very well, I lived and worked in Den Haag for 3 years in the late 90’s. A wonderful country that utterly spoiled me for cycling in many ways, but it’s a place I feel I already know well and getting there would still involve a day or two at either end of the trip from Devon.
A few general bits of advice. You'll want a decent bike planner. I recommend
Cycle.Travel. Lots of useful features for long distance adventures. Distance is just a number. It's very helpful to know what 20,30,40 km
feels like. Ditto with climbing. A daily target is useful but the really important thing is doing that distance day after day after day. Better to not stretch ourselves. Extra energy can be spent exploring along the way or when we arrive rather than racing to a destination. Going solo, self motivation or self care can be important. I'd suggest knowing how to do some basic maintenance like fixing a puncture. It can save on some embarrassment.
Best of luck to you.
I’ve used Komoot for a couple of years but only recently discovered cycle.travel and have been using it for the past week or two to plan this trip and I’m figuring out how it works slowly. Very much enjoying it though and think it will become my go-to planner from now on.
Maintenance wise I’m fairly well set. I’ve built a couple of bikes up from components in the past and just put new bottom bracket/crankset etc on my road bike. Puncture fixing has long been a staple of life and I carry a decent kit of tools/parts as needed when I’m out.
Thanks for taking the time to put so much great information and suggestions down, it’s really much appreciated.
Jay.