Think this way: postgress db is just part of immich. That's it - separate your services into logical units.

That's actually makes more sense to do at home lab. Bringing down your main DB breaks a lot of your services. By separating - only part would be broken.

My postgress db lives in the same docker compose file where immich is. If I decide to delete immich - it's very simple to run "docker compose down" and delete folders. :)

Ask yourself what your "job" in the homelab should be: do you want to manage what apps are available or do you want to be a DB admin? Because if you are sharing DB-containers between multiple applications, then you've basically signed up to checking the release notes of each release of each involved app closely to check for changes like this.

Treating "immich+postgres+redis+..." as a single unit that you deploy and upgrade together makes everything simpler at the (probably small) cost of requiring some more resources. But even on a 4GB-ram RPi that's unlikely to become the primary issue soon.