Great – that does eliminate a lot of things. 
I’m not sure how RoonBridge behaves on MoodeOS, since it’s optimised for well eh… Moode. You may want to go the DietPi-route on a spare SD card – it’s the best/most efficient way of running RoonBridge on a Pi.
EDIT: Just to add a datapoint: I have a Pi in my main system that is playing PCM192 for hours daily. It is outputting through SPDIF though – maybe (speculation alert!) since the Pi’s USB and Ethernet are still sharing the same (USB) bus, the higher network traffic and USB output are reaching the Pi’s limits – even though that does not rhyme with you saying DSD128 was fine with Roon 1.2. Anyway – if you could try DietPi: let us know how it goes!