Just from someone who has dug too deep trying to figure out what it would take to make this happen, KWin on Wayland right now does not support graphics tablets

Just from someone who has dug too deep trying to figure out what it would take to make this happen, KWin on Wayland right now does not support graphics tablets at all. It needs to be wired up to support them still. And we still don't have a Wayland input KCM, but I think someone is working on that for 5.13. I think. That being said, I have a Monoprice tablet I use through libinput. I used the wacom driver for a bit but libinput gives me enough to do what I want (I have to say having to do matrix math to constrain the screen size so the tablet is in 1 to 1 correlation to the screen was a bit rough on the UX front). Libinput is pretty feature complete on tablet support in the basic use cases, it is just missing a bunch of the tunables from xsetwacom. The Digimend project has done an amazing job providing driver support for a lot of non-wacom tablets as well. It will be interesting if its Wacom tablets that are the pain point on Wayland. I don't have or know anyone with one to confirm or deny if the libinput support is up to par. If I had the confidence to know what I'm doing (and I don't, I've never touched the Kwin / Wayland / input stacks on Linux at all) I'd try wiring in tablets to Kwin, but you probably want that first - which is still at least a release or two out at best - before you even think about trying to port your KDE Painting app to Wayland.