Just to say you can have a 'mixing layer' as such as a PIP. You just need to sub mix some inputs on a phantom Destination then bring that Destination in as a source for the PIP so you can fade sources in it.
We too like to use PDS-4K (SDI) quite extensively for smaller events as a simple screen switcher or LED scaler. Too bad the truthful name should be PDS UHD, though. Luckily there are bigger systems for larger projects but still, it’s kind of confusing that it does not actually do 4096x2160…