This is pretty much exactly what i do.
I only want to track the stems from the devices via USB once i’m ready.
That’s why i use VB-Audio Matrix. It doesn’t have to run all the time and i just use it when i need it. Feel free to have a look at my guide if you want and then you can decide if you want to give it a try or not.
USB Storage Mode will hopefully make it’s way to the Play+ as well:
Another very simple option that i have used in the past it to record the USB Tracks to AUM on the iPad and use the stems from there, this allows me to do some simple processing already before i bring all the tracks into my main DAW.
Lots of options, lots of roads to Rome 