Exactly what happened to me too! Been using a Sway setup on NixOS for many many years, and I was just curious to try out Plasma 6. After a small config change, I had the desktop up and running, and I was impressed how it felt like. You can even use plasma-manager to store your KDE settings to a nix configuration, which make it easy to have a unified configuration across different computers.

https://github.com/pjones/plasma-manager