+1 syncthing too. Over the years it has gotten rock solid for me. I use it between all my machines and phone. With various 1 to many, 1 to 1 and one way or two way sync.

One of my machine is always on and is backuped to the cloud everyday. Effectively making a backup of all my devices at once.

Better yet, since syncthing can also save staggered copies of modified files, a rm -rf * will be synced but the backup machine will still have copy from about 10s ago. This saved me a few times.

Once I realized that I had deleted a file a few months later. And I found it into the syncthing backup, itself within the cloud backup.

Very freeing.