Personally I wouldn't get too hung up on weight, for road touring as long as your sensible about what you pack then the extra kilo of a rack/panniers vs soft bags really won't matter at all. The more pertinent question is above, can you actually fit a rack to a TCR? if so, then will it be cheaper to buy a rack/pannier combo or soft bags? Like I mentioned in my first post as well the shape/size of your frame will also dictate what bags will ultimately fit as well.
Another thing that's come through this thread is how many good manufacturers there are in the UK, all the brands mentioned, Apidura, Alpkit and Restrap all make really good gear and I have stuff from all of those brands.
Lastly that route sounds really interesting, can I just add a thought in? Your planning on diverting down to Lake Bled, which is a lovely place and it does have a really good campsite as well. You will be in the middle of tourist season though and Bled can be dreadfully busy and the roads around are not too much fun. Have you looked into the Sôca Valley as an alternative? Turn South a little earlier go through kranjska Gora and then over the Vršič pass. From there you can head into Bovec and then down to Kobarid and onwards to Nova Gorcia. A stunning ride through one of the most beautiful parts of Slovenia. It will also be busy, but there's lots of really good campsites and if the weather is hot you can cool off in the Sôca river itself. I'm planning to pass this way myself, but over the mountains themselves rather than the road. Just a thought any way, no matter what you do you can't go far wrong in Slovenia.
Here's a link to when I rode through Slovenia on one of my first ever Bikepacking trips back in 2020 when I carried far too much kit on a bike that was hopelessly undergeared. A Fantastic learning experience though in an amazing country which is why I'm quite keen to go back next year.
https://www.cyclechat.net/threads/slovenia-bikepacking-trip-2020.264075/