How massive of an undertaking is this going to be? There is a mention they think they could get it done in 1-2 releases. https://pgpedia.info/postgresql-versions/index.html they seem to release once a year from what I can deduct. 1-2 years, not bad. I wonder how many people would work on this (full time/part time) and how many resource hours would go into this.

I wonder which layer would benefit the most.

You connect as pg_client to pg_server, you make a request (SQL query). It has to get tokenized/query planned, then probably some detection layer that already exists that says "do this all in a single thread or spread it across multiple". How does a single threaded process that spawns multiple threads -> a multi-threaded process benefit in these situations given the overhead of having to sync/message in between threads messages back and forth?