Some results on experiments that I just completed.

Fusion 12.1.2 on Big Sur: No VMware dhcpd processes fired off by Fusion on macOS. The custom network dialog does not show an option to enable/disable use of DHCP for any custom networks.

Fusion 12.2.6 on Monterey: VMware dhcpd processes are seen on macOS. The option to enable DHCP for custom networks appears for creation of Fusion custom networks.

So unless someone can come up with a way at the macOS level to disable DHCP for one of the VMware networking segments on Big Sur, I'd say that it's not going to be easy to disable DHCP the way you want.

If you can, I'd recommend upgrading to Monterey and Fusion 12.2.6. 

Side note: perhaps this is one of the reasons why Fusion dropped support for Big Sur in Fusion 12.2?? Apple matured their APIs to allow VMware to take control of DHCP on the Fusion-created subnets?