That's my point. I hate systems that require a phone number, as they usually mean that I have a substandard experience when I'm not on my phone and I can't sign my children up so that we have a general chat tool.

The only option ends up being massively over the top team style chats like Rocketchat, Mattermost, Discord, or Slack. So we end up back on Hangouts.

A bit shit for general family conversation.

[Edit] If they do allow signing up/in with a username then I'll probably be all over it. That would be awesome news.

No. Threema does not require a phone number (it uses one for the registration verification, but your account is not linked to that number).