I use syncthing. Love it love it live it!

Syncthing + Raspberry Pi + Large SD Card + my other devices = good enough for me. It is a decentralized system in two respects 1) no corporation's remote siloing and hoarding of your data since you are the "cloud" and 2) the system of Syncthing is also inherently decentralized requiring no central server (self hosted or not) as a critical point of failure to maintain a group file sync. I use my raspberry pi like a "server" but architecturally it is just another file sync device in a file share between devices, it isn't a traditional server in any strict sense of the meaning.

Another vote for SyncThing.

I've been using it for more than 8 years, just awesome.

There's a blockchain version called Filecoin that is basically a combination of torrent file sharing mechanics stored in decentralized locations. The "coin" part is their workaround to sort of define the equity of hosting.

I haven't used it, just heard about it recently and it seems interesting.

Edit: Now that I think about it, it's just big Syncthing on the chain.

If you want some sort of Web UI, checkout Nextcloud. To me it's a bit bloated but it is still the most mature service there is. It even has its own client.

SFTPGo is also great, albeit somewhat lacking in terms of features.