I highly recommend switching to GOS, it is wayyy better than iOS UX-wise and obviously better privsec and freedom.
One thing that I had to do when I first got GOS, to get a better experience, was find all the Open Source apps that I needed. Otherwise, it looks rather bland and the apps are mid. Once you find the right apps and launcher, everything works much better.
On the bright side, Messages works without linking to a Google account
When I first tried last fall I had it working for a few weeks then it stopped entirely delivering messages and I fell back to SMS only. After the recent system updates and enabling the ICC option it has been working well for me.
The official page explains briefly, https://grapheneos.org/usage#rcs
There is a very long discussion threat going back several years that is now considered resolved, which seems to be the case for me. https://discuss.grapheneos.org/d/1353-using-rcs-with-google-...
In the last week or so, multiple people have told me they cannot text me. I found that I was getting a "verification limit exceeded" error (perhaps because of my unusual behavior of usually being at work or at home, both which have known wifi networks, and sending maybe half a dozen texts any day?). I got the error to go away for half a day and they were still unable to message during that time, and now that I have it disabled I still appear as online on RCS (yet still unreachable?) so they still cannot message me lol.
I've been on the other end many times across multiple Android devices across multiple years, being able to send messages to some RCS users, being unable to send messages to other RCS users, not being able to receive messages in group chats entirely comprised of Android users, etc.
SMS/MMS: Handled by carriers, you can send messages to people who are offline and they'll get the messages when they turn their phone back on.
Telegram/FbMessenger/Whatsapp/etc: Handled by individual corporations, you can send messages to people who are offline and they'll get the messages when they turn their their device on.
RCS: Handled by both Google and carriers at the same time for some reason, maybe 80% chance of being able to send a message to somebody who's online, let alone offline.
I'm sure there are multiple reasons it was challenging, but Google and friends have not risen to the occasion at all. Truly a garbage protocol.
RCS I didn't even bother to set up. I don't want to use yet another system. If people want to reach me they have WhatsApp, Signal or Telegram to choose from.