I mostly agree with @shadejinx. I would add that when editing the same note on two separate devices syncthing will at least fail kind of gracefully in that you’ll whatever.md will still be there but you’ll see an additional whatever-conflict-hash.md along side it so you can easily fix it up. Synctrayzor for windows will give you a nice notification and UI with which to resolve.

Nextcloud is great but it’s a real behemoth. Loads of stuff you don’t need.

IDK what you mean exactly but I sync between computers and devices just fine.