wow the signal usernames design is good actually:
- username is exclusively an "adding you" thing
- this is because they are one way of getting an invitation to chat with you
- there's also randomized and resettable links that can be shared for this
- usernames must always have discriminator numbers, which can be freely selected
- usernames aren't shown to anyone you are in chats with
- by default phone numbers are no longer shared with chats

this seems to have basically Fixed it