Dude! [couldn't resist]. I've got an old (1st-gen) Apple TV 4K I already owned feeding my WiiM Ultra, and I'm using it to AirPlay into a WiiM speaker Group. I could've used AirPlay to go to the other speaker in the WiiM group, but they're in overlapping zones and WiiM's got audibly tighter sync.
Perhaps our two examples supports WiiM's decision to leave the AirPlay 2 receiver out?
Your use case sort of seems like the long way around the block for an inaudible upgrade - you might get better iPhone battery just going Bluetooth direct to the Ultra. I betcha a dollar you wouldn't hear the difference in a blind listening test

.
That said, leveraging the old AirPort Express is pretty cool and a useful audio path to have feeding a preamp!