You make a really good point about trading Sqlite write limits to the limited number of Postgres connections. My comment is mostly about having to run a proxy, which is another service that ends up being a liability. Regardless, if you need multi-writers, then the obvious solution is Postgres.

"Both happening on the same nodes and no isolation whatsoever. More things can go wrong and will require more tooling to disambiguate between different potential failures."

As a Postgres fan, the extra Postgres service to me is a far bigger liability than Sqlite. The east-west networking is true for Postgres replication as well, so it's a wash to me.