Linux

Others have said it, but SyncThing all the way. Open source, been around for a decade, battle tested, no cloud, full control over everything.

I didn't see this mentioned, but you can also tell KeePass to auto reload the database if the file gets updated elsewhere. Makes it so you can run the same KeePass database on multiple devices with live/realtime updates. I've used this setup instead of vaultwarden/passbolt on several IT teams to keep the important stuff separate from the normal systems. It's not on by default usually, but right in the Basic Settings page under File Management.

I have KeePass+SyncThing on 3 laptops, 2 androids, and a home server. If I add a password to one of my androids while I'm out and about (and I have cell data), next time I sit down at my desk it's already available. Vice versa works, too. If my home server dies, the other devices don't care and keep syncing amongst themselves. I think I've had some version of this setup going since SyncThing released, I can't imagine using anything else.

Do note that since there is no cloud or infrastructure behind it, sync conflicts do happen when a device in the network goes offline for a while. It'll never get rid of files if there's an error syncing, but instead create a second copy with a timestamped filename. If this happens to your password db file, KeePass can then merge the two copies together and sort things out mostly automatically. Over the many years I've been using this, it doesn't happen as often when you're the only person using any of the devices that sync. It can happen a lot when you share the setup with someone else, though.