DramaticHusky
I installed Graphene for the first time yesterday on a 7 Pro and RCS is working. It's been less than 24 hours but it seems fine and I hope it stays that way.
You probably saw some of the same reddit posts I did while researching. There's plenty about RCS in this forum including this very lengthy thread. As you noticed, some folks struggled while others had it working right away.
You may need to adjust permissions where needed, in particular READ_DEVICE_IDENTIFIER that can be modified in adb shell. Also, I think it helps if RCS was already setup and working in stock Android before being replaced by Graphene. In fact, Graphene's phone app imported Google Phone settings for call forwarding which saved me extra setup. As the docs explain, "GrapheneOS extracts CarrierConfigs, APNs, modem configurations, Visual Voicemail configurations and MMS configurations from the stock operating system..."
If RCS is important to you, install sandboxed Google Play Services + dependencies from the Graphene app store. This is a small compromise for me because I need Google Voice and want the option of installing Pixel Camera if Gcam mods don't work out.