Matrix is great. Just remember that the Matrix threat model isn't the Signal threat model: you're usually telling a Matrix instance --- or, really, anyone who can compromise or suborn that instance --- a lot more about your communication patterns than you are with Signal.

Matrix, right now, is a lot more amenable to the kinds of messaging that people on HN tend to want to do than Signal is. The problematic thing is that HN people tend to believe that their workflows are (1) the most important and (2) the ones with the most sophisticated threat models. Neither are true; (2) is very un-true.

For, like, talking to team members about a shared dev project, I'd always use Matrix in preference to Signal --- of course, for that kind of work, what I'd really do is just use Slack or Discord. Which gets at something about what HN wants from Signal.

If the phone number requirement is the only part you dislike, buy a tourist Sim card with cash. The greatest lie of online identity is that phone numbers are tied to individuals forever.