I think I use Syncthing more than any other tool. I have a bunch of different computers, and all of them are running a Syncthing server, all hooked in to the same folders, all sharing the same documents. I have it running on my GrapheneOS phone too, so my photos folder gets shared as well.
It can be kinda fiddly to set up the sharing, making sure that you point the shared folders at the right place on your system, but once you’ve got it dialed in it’s invaluable.
For example; it’s where I keep my Calibre library, so no matter which of my computers I’m on, I can open Calibre, drop a book in, and know that it’ll be ready to load onto my Kobo. I do a weekly radio show, so I keep all of the documentation around that in a folder that I work from locally, whether I’m on my MacBook or Linux desktop.
The only downside to it is that (as far as I can tell) you can’t store everything on one device to download to others as you need (like iCloud Drive or Dropbox), so if your Syncthing folder takes up 30Gb on your 2Tb server, it’ll also take up 30Gb on your 128Gb phone. So it does mean having to be a little judicious with what you drop in there.
Basically, I love Syncthing. It means that I have access to everything I need access to, without having to shell out money each month to rent space from a cloud provider. And because I have all of my devices sharing all the folders with all the others, even if one drops offline, the others still get updated damn near immediately.
Ah, that’ll be why it feels rock solid then…