I don't really have the expertise to contribute to a project like Replicant, but I'm very interested in it nonetheless (and fully support it obviously).
The other phones on the market that I know of trying to use free software are Pinephone and the Librem 5. Neither one, AFAIK, is considered fully free (which I assume is why the FSF hasn't supported them). So I was curious: what free features don't these phones have that Replicant is trying to create?
If I had to guess,the main obstacles would be Bluetooth compatibility and the fact that the OS has to interface with the Qualcomm chip (which I believe are both patented and propeietary). But I'm curious about the specifics from people who actually know what they're talking about.
Did you read this article?
I think it answers a lot of what you are asking. If, after reading, you have further questions, please do ask.