All the environment variables are documented in the /etc/egroupware-docker/docker-compose.override.yml file, were you can set them.
Other tuning tips are in the wiki page Tuning EGroupware for higher number of users.
That does not include the pm_* settings, which are normally enough for most use-cases, thought you can use the same technic described in the above wiki page, to copy the config file out of the container, modify it for your needs and mount it again into the container, so the modifications are not lost with each new container version.
spawning 32 children, there are 0 idle, and 75 total children
This is quite a high number!
Either we’re are talking about a huge number of users (thousands), or something is slowing down you’re requests a lot. As longer a request takes to process as more processes you need for a constant request rate.