This concern is a bit overblown here. There are 23 KDE patches at the moment, across multiple packages. kwin itself has 5, plasma-desktop has 4. Each one has an extremely limited scope - mostly one line changes related to paths rather than visible functionality.

If you have a specific issue under nixos, it's going to be fairly easy to see if it's very unlikely to be caused by custom patches.

From my experience, reproducibility is king. If you can give a formula to reproduce a bug, even if it is obscure, it makes it so much easier to track down.

That can be up to and including a downloadable VM image that shows the issue.