I use syncthing for notes across iOS and macOS devices and I wouldn't say it's great. It works most of the time until it doesn't, until one client all of a sudden stops syncing and then you gotta figure out where it's hanging. Couple of days ago it was the macOS syncthing app. Just wouldn't start, wouldn't show an error. Usually it's the iOS helper app "Möbius Sync" not actually syncing in the background. It's annoying but still better than using a cloud provider.
I suspect the experience is better on Android.
It is much better on Android
Note that work on the official android app is discontinued
https://github.com/syncthing/syncthing-android --- "Discontinued This app is discontinued. The last release on Github and F-Droid will happen with the December 2024 Syncthing version."
Works with all file formats, from photos and movies to text files. Cross platform: Linux, Windows, Android, probably also Mac and BSD.
Update: And it's end-to-end encrypted and free, open-source software.