Like almost everyone, then you are setting your pool size way to high. Don't let your postgresql server do the connection pooling. If you do then it severely hurts your performance.
The optimal setting for how many concurrent connection to postgresql is
connections = ((core_count * 2) + effective_spindle_count)
That means that if you are running your database on a 2 core server, then your total pool size from pgbouncer should be no more than 5. Pgbouncer is a lot better at handling pooling than postgresql, so let it do that.
So, leave the max_connections in my postgresql.conf to it's default 100 (no reason to change as it is a max. Also this should always be higher than what your application needs as some logging, admin and backup processes needs connections as well)
And in your pgbouncer.ini file set
max_db_connections=5
default_pool_size=5
max_client_conn=400
For more information https://www.percona.com/blog/2018/06/27/scaling-postgresql-with-pgbouncer-you-may-need-a-connection-pooler-sooner-than-you-expect/
