Yeah the phone folder is send only and the server syncthing-ext folder is receive only.
I never really find myself deleting stuff because I don't take that many pictures/videos, but my wife is the one always running out of space. I've told her when she sees her album in immich it's safe to delete from the phone.
I'm not sure how you'd automate deleting stuff while making sure you're not deleting items that haven't been sent yet... but, it's not really something I'm interested in because I don't have the need.
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.