Lots of people have moved away from raid entirely because of some of these issues. There are alternatives these days. For example mergerfs or the ZFS file system.

So… like Immich?

Yep, because they sound plausible.

I wouldn’t know. All I am saying is that Syncthing would not work for this purpose.

Syncthing works on a file level basis. If files are changed on both devices at the same time, it will have sync conflicts.