Still, about 0.02 dB worth of filter ripple is too much. I'd check for the presence of resampling / try at a 48 kHz multiple (I doubt anyone in professional circles would be running their DAC at 44.1 these days).
Speaking of sample rates, you took DAC FR at 44.1 kHz and ADC FR at 192 kHz, was that intentional? It certainly is a good idea to be running an AK55xx at minimum 192, even better 384 kHz, mostly because their filter design for single/quad speed recording is a bit silly (+/-0.03 dB of periodic ripple in an FIR filter even in a flagship ADC, c'mon).
Probably the input stage / ADC driver breaking a bit of a sweat. Hard to tell which, almost +24 dBu is quite high after all. A distortion sweep may or may not provide some clues.
Having an OCXO is more about long-term stability / drift anyway. It's quite hilariously overkill for audio, but I guess if you were to feed its 10 MHz reference input from a GPSDO there would never be any doubt about things being in sync, even without the need for a central master clock (which is all fine and dandy as long as all the equipment physically is within the same studio, but you know how it is these days).
GPSDOs are fairly inexpensive, you can get one for 5-10% of what this converter costs. You do need a spot with GPS reception, and obtaining proper sync can take a few hours.