Audio: Way to disable HDMI or set USB as default?

[hatenick]

The alsactl save command is not used.

I'm not able to repro your issue by turning off my USB DAC then turning it back on a few minutes later.
moOde 930 | Pi-3B+ | Allo Revolution DAC
...

After some quick checking it seems that RPI3 and RPI5 are very different in terms of both USB and HDMI in both silicon-wise and kernel-drivers-wise. This might explain the ALSA driver index jump, and moOde changing of output due to some behavior change of the drivers.