I used to tour, and general cycle on an oldish Giant hybrid.
Fitted me fine, not super fast, but even with light luggage, super fast isn't where touring is generally at
After twelve years and thousands of miles I thought I was due an upgrade.
After reading all the stuff on t'internet about gravel and adventure bike (I go about 80% on road 20% off) I was ready to drop some cash on a bespoke steed, from my lbs, who does builds.
However after discussing it all with them, we came to the conclusion that a more modern version of what I was already riding would probs suit better.
So I got anothe Giant hybrid, through the lbs - disc brakes (I do like) and bar ends etc.
So lbs passed up a chance to make some good money, but I got what I needed.
It does make it slightly more nickable, I guess. But the smoother ride (and better braking) are worth that slight extra worry..
I still hang all my scruffy luggage and washing off it when on tour, so maybe that helps deter would be thieves
Re the knees, sounds like overdoing it on the hills.
A stretching programme which involves full extension of the knee joint might help.
But that's better done under professional supervision if there is a problem arising.
Sounds like you've been to some grand places as well.
Fixability in out of the way locations is key.
As other have said, it's the travel, not the speed that's the thing for most of us, and having a trusty steed is vital for that