Did you follow Google’s documentation to setup the postgresql instance? If you use Google’s documentation, can you share it please? The recommended settings are always part of Google’s documentation.
Here is a google document to create a PostgreSQL instance [1]
Do you see an improvement if you use the default connection amount limits, according to what is recommended from the documentation? The documentation also describes this error being caused by connection timeouts, have you found better results by increasing the connection life?