Thanks much for your inputs.
For now, I kluged it with my always-present startup script. For reference, my exact commands are:
/usr/bin/pactl set-default-sink 'alsa_output.pci-0000_00_1b.0.analog-stereo' &
/usr/bin/pactl set-default-source 'alsa_input.usb-Audio-Technica_ATR_USB_microphone-00.analog-stereo' &
I don’t trust these things to repeat their enumeration every boot so I got those full names with:
pactl list short sinks
pactl list short sources
I hate having to hack like this but hope this helps someone else.