"Turns out that while JRiver is now presenting the native frequency (44.1, 192, whatever), the bit depth is fixed at 32."
You might be able change the bit depth with this sequence within JRiver:
Tools -->Options-->Device Settings-->Bitdepth
I'm running Windows 11, JRiver 28 and drivers which are specific for my DAC, but this sequence may get you to where you want to go.