I was in a similar situation as you. However, rather than bashing my head (figuratively) against the same wall, i just moved to a vps with more space. Granted, this was before using external volumes was as easy as it is now. But reading that your two main apps are Immich and Sync thing... there's two walls.
Listen, there are amazing workarounds and solutions offered by Cloudron and from others on this forum. But, I've never viewed any of this as "recommended and official"; it's more, "enough people refuse to/can't do the obvious, so how can we help them?" The obvious being - get a server or vps or whatever that has all the space you need. It's one reason I moved to ssdnodes (but just enough users have not had a good experience with them so this isn't an ad, I've just had excellent results with them and the 1.2tb server they offer.)