I am generally a fan of using as few moving parts as possible but if > 60-70% (2TB + a "few hundred GB") of your prod database are an append only audit log surely if would make sense to split that part into a separate DB server? Especially when you are using a hosted service. It sounds like both uptime and consistency requirements are very different between these two parts of the production data.
Postgres makes a lot of sense with append only tables. You can easily partition them by time (usually) and thus have an easy way to break up the index trees as well as using a cheap indexing scheme like BRIN and being able to just drop old chunks as they become irrelevant.
