I am +1 for auto-connect avahi-control and raw-usb to ghostscript-printer-app ,hplip-printer-app, gutenprint-printer-app and ipp-usb since both are needed for expected operation of these snaps as also discussed here.
I am +1 as well for auto-connect hardware-observe to ipp-usb, hplip-printer-app and ghostscript-printer-app since it seems to be the best way to achieve UDEV observation based on this discussion.
Can other @reviewers please vote?