This I’m not so sure about. I have two Mac Pro 5,1:
2x 3.46ghz Xeon X5690 (12 cores), 128GB ram, NVME SSD, Radeon RX580 8GB
1x 3.46ghz Xeon X5690 (6 cores), 32GB ram, SSD, Radeon RX580 8GB
Both use Big Sur 11.6 (and OC 0.7.4)
In both cases the CPUs do nearly nothing while riding in Zwift (use iStatmenus to see it). The GPU gets hammered while the first CPU core is about 30% and the others all idle.
Both of those machines are fine in normal riding with the 4K ultra setting but group rides with 100 riders close together are not smooth using HDMI 4K output to a 55 inch TV.
Looks like Zwift needs hefty graphics cards. Can anyone find me a brand new Radeon VII? (aka winning lotto).
For the OP, could you think about something like a refurbished/second hand Mac Pro 7,1 with a lower level CPU then put one of the latest and most powerful AMD GPUs in it? You’ll have a computer that you can upgrade for years to come especially in storage, GPUs and CPU. There will always be some of those types of machines going cheap, just keep a watch for them.
If you have high end projectors (which are costly) then $$$ probably not a concern.