DARbarian

No app I know of, but imginn.com still works for me with public accounts

Nope. LibreWolf & Waterfox are top-tier, Zen and Mullvad Browsers aren’t bad at all

I went through this same process recently. Since Apple was finally implementing RCS, I was looking forward to my iPhone friends complaining less, but unfortunately it seems like Google pretty much has an iron grip on RCS as even with Samsung Messaging or Beeper / Sunbird, I’m pretty sure you need a Google Account