Well that's the thing, you just don't know what happens once you let Signal send notifications via Apple/Google - clearly they get them plaintext, and who knows if they're retained and subpoena-able directly from Apple/Google. The leak via notifications DB not being cleaned up is just the shot across the bow. You pay a price for convenience.
Anyway, I'm not OP, and they have a mad setup (XMPP via Tor) which is a flaky solution most people wouldn't go for. In general, if you're not going to such extreme measures of hiding among the crowd of Tor users to mask your metadata, you're better off directly connecting and hiding among the crowd of Signal users, rather than hosting your own instance.