I’m using a custom ROM with microG, which potentially means no RCS since it is only available through Google Messages which doesn’t work with microG.

This is a common misconception. Android just doesn’t let you access the necessary low level stuff to talk to carrier RCS services. If you’re already using a custom ROM that may not be a problem. Here’s a third party RCS demo app: https://github.com/Hirohumi/RustyRcs

I don’t know how actually usable this is though, or if there’s anything else that’s usable that’s not Google Messages.