Sadly, a centralized, monetized space-based signaling system seems less like pie-in-the-sky to me at this point than does any particular decentralized platform gaining sufficient network effect to become a widely used standard.

Any good ones? Last I "used" was BitMessage I ran it for maybe a week and never looked back.

Try Matrix.

Ah didn't even occur to me.. of course. I have been running my own server for a few months using Oracle free tier. Think I'll stick to matrix.org going forwards.