Regarding a), a sharded postgresql (i.e. with citus data) can easily accomodate that workflow with just a tiny bit of extra overhead.

Re c) this strongly depends on the usecase, I've seen companies use a) to avoid split-brain problems and having to manage two data pipelines to great success at similar scales. You might find https://www.youtube.com/watch?v=NVl9_6J1G60 interesting.