Syncthing or nextcloud

I use Syncthing for this. I have a few Linux boxes at home (a pair of servers and a laptop), a Macbook, and two Android devices (phone and tablet) that are all hooked together with a shared directory. Also, even though you don’t have to have them, I set up Syncthing Discovery and Relay servers on one of my public boxen to help the community out.

Syncthing is robust, used it for my encrypted passwords (KeepassXD) on home network, not vouching for its safety just a use case

If you don’t need to access them through a web browser, I would likely suggest SyncThing, like @GravitySpoiled@lemmy.ml did in another comment. Nextcloud would be way overkill for this usecase.

Git would require you to manually commit and pull any changes you make.

Perhaps this is close enough as a use case that it can be of help:

I’ve kept a folder and subfolders of notes on my phone for years. I used to sync them to a Nextcloud server until that became too unwieldy for my single-user needs. Now I just keep the files synced across my devices with Syncthing.

It has optional file versioning and simple conflict handling, not always instant syncing but close enough for my daily needs.

Seafile or nextcloud are my choices. I like seafile because it has an official and documented way to install it but nextcloud works well too just installing it can be tricky. One thing I like about sea file is they have a remote filesystem app that supports Linux and works better than nextcloud and webdav

Another vote for nextcloud.

Or synchthing if you want something that is serverless, but does not support sync on demand.

Git would be my choice.