That is indeed really cool, thanks for sharing! It inspired me to install Debian Bookworm(x desktop + vnc) and hqplayer 5 and it is indeed running smoothly up to 768 pcm(maximum of my Chord DAC). I can't get DSD 256 running without hickups though (DSD128 is fine). If you say headless, are you having an X desktop installed and are you accessing it via VNC? I prefer to run Debian in minimal configurationm without X/GUI, but the HQPlayer app seems to rely heavily on GUI depdencies. I tried HQplayer embedded on the RPI5 too, since that might perform even better. But it isn't starting, assuming it is still for RPI4 only.
I am also wondering which filters you are using for DSD 256? Thanks