I understand that you want to send only some specific surveys (of the 20) from specific events.
If that's the case, in my experience your best shot is to activate and enter the survey queue of each participant, and there enter the surveys you want to send and copy+send the unique link of each survey.
It's a little tedious but effective in my opinion.
Be aware of deactivating the option that moves automatically the participant to the next survey when they complete/send the survey.
Maybe there's a better way I'm not aware of.
Hope this helps
Nacho