To fix multiroom/mdns run and restart the volumio service
sudo apt install libnss-mdns
volumio vrestart
You still have the wrong systemd unit file. Please try steps from Volumio Debian Buster Beta - Raspi images debugging - #380 by ashthespy
Confirm that cat /lib/systemd/system/shairport-sync.service has similar looking things as the file from the repo.