Or more precisely, Signal will only use fallback notification services if Google Play Services is unavailable at the time Signal is installed.

I just wanted to clarify that it's not necessary to install Signal once with Play Services and again without. I've never had Play Services on any of my phones, but was able to start using Signal without issue.

Okay, I didn't know those were things you were concerned about :).

On the lack of self-hosting and federation, I don't like it either, but it is a deliberate choice to allow faster evolution. That post, from ten years ago, contends that federated protocols ossify too easily, while centralized can grow and adapt (relevant quote: "So while it’s nice that I’m able to host my own email, that’s also the reason why my email isn’t end-to-end encrypted, and probably never will be"). This does give me concern that Signal will not last in the long-term, but for secure communication right now, it's hard to beat.

As for US hosting, that's another thing I don't like, but they are structured to mitigate that issue. Their protocol, which has been independently audited several times, ensures neither the Signal Foundation nor any intermediaries can see user activity. They have been subpoenaed before, and have only been able to provide the date a phone number was first registered, nothing more. And since the Signal Foundation is a non-profit, they have no incentive to enshittify to turn profitable. So even under US government threat, they can't see what you're doing and have no ability nor reason to change that.

Anyway, that's why I settled on using Signal, despite its imperfections. You may be happier with Matrix; I stayed away from that because it's more work and less secure (as I understand, group chats only have message content encrypted, with metadata like users and message times being unencrypted), but its federated nature may let it outlast Signal. And there are many Matrix clients in F-Droid that don't require Google Play Services.