Syncthing is a wonderful open source project that I thoroughly enjoyed using when my mobile device was on Android. Everything I needed was synced, all the time. Great performance, too. And it was really easy to set up syncing of a particular folder with random people for various projects.

I switched to an iPhone for greater privacy, and was crushed to find Syncthing doesn't support iOS.

I see that 5-6 years later they still don't. Sad, given the iPhone has a majority market share in the US (60%).

I use Nextcloud and it's....okay. There's some weirdness where files I've specified should be always available somehow aren't, but luckily the keepass client keeps an internal/backup copy and is smart enough to open that.

iCloud is just...a rolling dumpster fire. And easily the most frustrating thing about it is that there's no way to set up an iPhone without it immediately trying to sync everything to iCloud, like say, your iMessage encryption keys...