Super strangely, I'm not sure what I did to resolve this but the issue has now gone away.

A few of the steps I followed including trying to install gRPC through Xcode dependency manager (that didn't seem to resolve the issue), then removing Firebase and gRPC from Xcode dependency manager (then also closing Xcode and restarting my Mac), reinstalling Firebase through Xcode. Through my various attempts I also cleared (and sometimes accepted) all uncommitted git changes etc, I also created a rollback branch on GitHub. None of these individually seemed to solve the issue, but when done all together, things seemed to spur back into action.

Thanks for your help @Jatin.