I’m not sure how you’d automate deleting stuff while making sure you’re not deleting items that haven’t been sent yet
Yeah this will be very tricky, I'm thinking if I got over the initial push to Immich, I just delete manually from my phone when I see things have already moved to Immich and I don't need it anymore.
Another good practice (at least I’d like to think so), my syncthing-int which sends everything to /photo_libraries also sends everything to two other backup folders that are pinned to separate discs. So if I have a drive failure or multiple drive failures I’d still have a copy of all the photos/albums.
I will do that to ensure I've backed everything up somewhere in case an Immich Failure.