in that case, you’d be better by not using Google Messages. According to the discussion I linked there seem to be a few other proprietary RCS clients in the Play Store, other than Google’s and Samsung. Not sure of this myself, but it’s worth looking into it.

If you don’t want to install Google Play services, your best bet is trying your luck with any RCS client other than Google’s. Even Samsung’s (if it even works outside of Samsung phones) has a bigger chance of working without Google Services installed.

Once you find one that works on a degoogled Android, just follow the usual recommendations: install it in a separated profile, give it as little permissions as possible, maybe a VPN if you don’t want them to get your IP (although given that your RCS provider will probably be your ISP this might prove pointless), etc.

And remember to assume that it is not private at all and they are harvesting all your metadata. The encryption is proprietary too, so there’s that.

Edit: I just remembered that encryption is probably exclusive to Google Messages. So you’re screwed, I highly doubt Google Messages will work without Google Services.

I’m guessing that in the near future when Apple launches RCS, we will have more options in Android too. So just keep up with the RCS news.