Not a lot to say however, from what I have gathered in your testing of <10 people WQ worked however when turned of the entire playerbase the server very quickly crashes.
could it be possible to setup a building where the cultist is and only allowing a set number of people in there to interact with the cultist for a set time, for example
10 people can enter the building at once, and after 5 mins they are kicked out again with a 30 minute wait to re-enter the building.
obviously I don't know if it's the sheer number of people trying to submit at once that is causing issues however I think this would help reduce that problem,
the only problem I can see is if people have more than 5 of the pokemon to hand over, but I think that could be remedied by having a pc in there.
The cultists building could be a derelict pokemon centre