I'm part of several group chats that include all iMessage folks, other than myself.
Wasn't aware that iMessage and RCS were even group chat interoperable. That sounds promising, thanks.
Whatsapp works fine for me, including push notifications, and you can download the app from their website (no need for Play Store or Play Services). My suspicion (but it might well be wrong) is that Whatsapp sometimes blocks new account that were created on GOS, but if you already have registered your phone number on the iPhone you should be fine.
So far I don't have a WhatsApp-account, as this would only be my fallback if I couldn't get RCS to work, but if I decide to make the switch I will keep this in mind and create a WhatsApp-account beforehand.
Thanks a lot for your feedback! Guess I'll keep watching the forums until Pixel 10 support arrives and decide then..