I spent some time today trying to replace pg but I ran into an issue. When using an rds proxy with iam authentication, it seems to repeatedly retry authentication and eventually my lambda functions time out. If I switch to using regular credentials it works fine. Using the exact same options with pg+iam authentication also works fine which leads me to believe it's an issue with this project.
It was indeed the RDS Proxy. It appers to be very strict about the client_encoding parameter. Postgres.js was sending 'utf-8' which PostreSQL will understand, but RDS Proxy would just hang until this was changed to UTF8 (uppercase with no dash).
I'll open an issue on GitHub tomorrow.