I think if it comes to it, GrapheneOS will need to include some suggestions in a first-time setup wizard. Alternative text messaging clients like QKSMS for example have been discussed extensively on this forum, with various issues having been reported.
Next best thing to do for now might be for the community to talk about the solutions that work for them which may not be widely known. Reach out into Telegram/XDA/r-fossdroid groups etc. to find well maintained solutions from developers who are willing to address privacy/security concerns. The good alternatives can be added to the opening post for visibility and perhaps stickied.
The other thing is that GrapheneOS is an open-source project, but it may be the case that Google Messages is the most usable text messaging client available due to a lack of interest in that space. This is a subtle thing that can slightly fracture communities if the messaging is not clear.
