Makes sense, definitely if anonymity from conversation partners is desired then I can see how Signal's model falls short of your needs. I've also used Signal in the past for activist group chats, but in those cases my primary risk vector has been having messages read by someone outside of the group, people typically join those after being referred by someone they met face-to-face. I suppose there's always the risk of a wrecker slipping in, and having more layers of anonymity could reduce that downside risk. Hopefully this username approach is able to address your needs better! I really like the tool so would be stoked to see it cover additional use cases.

This is still bad, as any of them sees all the phones of members of the group. In Telegram you can hide your number.

Presumably if they introduced usernames they would also allow you to hide your number in a chat?

Absolutely, not to mention if you ever switch numbers you have to change the signal information. For me, its fine for verification, but I want an identifier I ever have to change.