Your texting conversations will just default to SMS/MMS even if the other party is capable of RCS. For me, this is no real detraction as I don't have need of any of the RCS features and SMS/MMS suit my needs fine. Backing up your texts on a phone with RCS enabled and restoring them on the graphene phone without RCS will still succeed, although I've noticed some sms apps(like Fossify Messages) won't sort them by timestamp properly, I'm assuming because it either can't read the RCS timestamp or it doesn't count it as an SMS. The content is there, it just isn't sorted by time received.

I use Signal for conversation with my family and it works seamlessly across Graphene and desktop. Just make a backup and restore it on the next phone you'lll be using signal on.

It's really not that hard to run it without Google apps/services. In my case, I lost my MTB tracking apps as all of them require Play Services and my Starbucks app does as well but everything else I need , including banking and medical work without issue. For everything else needing Play Services, I was able to find a suitable replacement(for instance, Paralino in place of Life360, as they offer a specifically degoogled APK).