You have 32GB of RAM. Use it! 
The pm.max_children value is probably still a bit low, given the amount of RAM you have in your server.
Found this article, which might be helpful:
Personally I use the calculation from C. Riegers guide. Nextcloud 23 Installationsanleitung mit Apache2 - Carsten Rieger IT-Services which lead to the following values on my server with 8GB of RAM:
pm.max_children = 281
pm.start_servers = 140
pm.min_spare_servers = 93
pm.max_spare_servers = 187