https://github.com/y-muller/personal_configs/tree/main/xkb My notes about customising the keyboard in Wayland. It's brief but should get you started. And if you look around the repo, you might find a few other ideas. ;)
I keep my keycaps blank. It's simpler that way. I made pictures, as above, to help at first.
Keychron ships from China even if you order on a site in Europe. But it is quick and all charges are already included. I like their keyboards. The V series is very nice for the price.
With the large number of languages you are working with, I think it is worth spending a bit of time creating your own layout. The AltGr layer is where you'll want most of your changes.
On Wayland this is not too difficult to do. On X11, skip the headache and modify the system file.
This is my current Alt layer, mostly for French, with Spanish also being easy enough. My main layer is mostly UK/US alphas. This is on a split 42 keys keyboard but the concept is the same.
If there are some characters that you use very rarely, don't forget you can probably get them with a Compose sequence. For example, I forgot the ß when I redesigned the layer, but I can still get it with Compose + s + s.
I previously switched from ISO to ANSI and it doesn't change much. If you make your own layout it doesn't matter. I think you have an extra key in ISO.
Keycaps? I use blanks. But there are a lot of EU vendors. I can point to a few later.___
Since I have an encoder on my newest keyboard, I try to find some use for it.
On the default layer, it moves the cursor left and right. I'm experimenting with inverting the control mod to move quickly word by word unless I press Ctrl. Pressing the encoder toggles a scrollwheel mode.
On other layers I have the obvious functions
It's fun to configure all this but then I find that I don't remember that it's there and don't use it much.
Here's a list of vendors. https://wiki.keyboard.gay/VENDORS.html#continental-europe I've used Delta Keys, Oblotsky and 42Keebs. All good.
Edit: another list https://kbd.news/vendors/Europe