Hi Robbie,
wow, that’s a nice setup for a church! They surely don’t have those budgets here in the Netherlands.
After genlocking the camera’s and making sure de mixer doesn’t synchronise the inputs again (FS off), check if you have deactivated DVE’s that can add to the delay. I haven’t used Atems yet, but our Panny mixers do add 1 frame while using PIP’s.
I don’t think it’s possible to genlock the wireless camera without 1 frame delay. Receivers I’ve seen always buffer the incoming video in order to sync them.