Thanks for your quick and kind advice. Managed to get it to work. You were right, setting the exact dhcp range and gateway per interface did the trick. Your remark was spot on! This setup is working now:
/etc/dnsmasq.d/00-dnsmasq.conf
# USB Gadget
interface=usb0 # USB interface
dhcp-range=set:usb0,192.168.77.2,192.168.77.21,255.255.255.0,24h
dhcp-option=usb0,3,192.168.77.1
address=/access.tardigrade/192.168.77.1
# Alias for this router
# Wifi-AP
interface=wlan0 # Access Point
dhcp-range=set:wlan0,192.168.79.2,192.168.79.21,255.255.255.0,24h
# Pool of IP addresses served via DHCP
dhcp-option=wlan0,3,192.168.79.1
Yes, you are right that this is indeed not really a Pi-hole issue. Nevertheless we are using the the pihole-specific tailored version of dnsmasq only. There is no other version of dnsmasq installed on our machine (if there was, it probably would cause further conflicts... or not work at all.)
Thanks again for your advice! This was really helpful.