We were recently testing b2evolution.net, piwigo and ZPC (also in different server environments).
Users were then ask to "play with them" and after some time they had to share which application they prefer.
For my surprise, they voted for ZPC. I say surprise, because I expected them to prefer frontend editing rather than an separate backend.
When it comes to picture management and performance: my impression is, that the quantity is unimport for ZPC. Your server will set the limit. If you keep the picture and album tags up to date you should have an easy living with ZPC. Pls look also for DAM: https://docs.kde.org/trunk5/en/extragear-graphics/digikam/using-dam.html
As ZPC is folder-/file- based - you'll always have the option to "hard code"

and you always have an real overview over the pictures and albums.
Furthermore: I was not able to find any hacked ZPC-site. But I found "others".
I think ZPC is a hidden champion and it is worth giving it a try.