Might want to ask in !opensource@lemmy.ml, since you'll probably get a wider audience than asking in a KDE-specific community.

Hey there! My note-taking app works on Linux, welcome to check it out: get-notes.com

Currently, there's no sync or mobile app, but that's in the works.

I use Joplin with a selfhosted sync server but tbh I almost never use it on Android because it's just so cumbersome. Would be up for reccs as well. There's Nextcloud Notes (no desktop, only browser afaik) and a lot of people seem to be using rabdom Markdown note editors and syncing that with Syncthing but I've had bad experiences with that kn the past (stuff not being synced, constant conflicts etc).