By visiting Allen and Heath's website, one can determine the manufacturer assumes a wireless router with at least one LAN ethernet jack would be available for use with the Qu-32 soundboard.
The idea proposed in the posted question of simply substituting the iMac's macOS Internet Sharing for the wireless router does not work because a different LAN is created for each chosen physical port. Also, the iMac's WiFi can only be assigned to one LAN at a time
For example, if a second WiFi adaptor (as a physical port) was add to the iMac, then by employing macOS Internet Sharing, the iMac's ethernet physical port and the wireless access point provided by the second WiFi adaptor would exist on different LANs.
Using the existing wireless access point may be possible provided which TCP/IP ports to forward in the iMac can be determined and that the chosen ports would not conflict with ports needed by the iMac for proper functioning of the iMac. There have already been answered questions explaining how to instruct a Mac to port forward. Below are two such questions. Unfortunately, the answers probably are not as dumbed down as much as requested.
A simple solution would be to acquire a wireless router with at least one LAN ethernet port, as shown in the Qu Series Reference Guide. In this case, the only reason to use the iMac's macOS Internet Sharing is to allow devices in the same LAN as the Qu-32 soundboard access to the rest of the internet. Below are two modification to the instructions given in the Qu Series Reference Guide that would allow such devices access to the internet.
Turn on the router functions, such as DHCP, in the wireless router and use the wireless router as a router, wireless access point and ethernet switch. The iMac would be connected to the WAN ethernet jack on the router. All remaining ethernet connections would be to the LAN jacks in the router. Here, the router functions, such as DHCP, would be supplied by both the iMac and wireless router. There would be two LANs. The first LAN would be between the iMac and the wireless router and the second would be between the wireless router and the other devices.
Turn off the router functions, such as DHCP, in the wireless router and just use the wireless router as a wireless access point and ethernet switch. All ethernet connections (including the iMac) would be to the LAN jacks in the router. Here, the router functions such as DHCP would be supplied by the iMac. This configuration allows the iMac to exist in the same LAN as the Qu-32 soundboard.
General. Qu. SQ. Sending CC MIDI messages from an app over TCP/IP to control QU/SQ
Share MIDI information over a network in Audio MIDI Setup on Mac
TCP and UDP ports used by Apple software products
Qu-32 38 IN /28 OUT Digital Mixer
Qu CHROME EDITION Getting Started Guide