If you are willing and able to put together the type of system described in the OP (a workstation-class PC, with multiple discrete GPUs and often multiple power

If you are willing and able to put together the type of system described in the OP (a workstation-class PC, with multiple discrete GPUs and often multiple power supplies), a Mac never makes sense. There are hardware options available at essentially every price point that beat (in some cases drastically) the performance and memory capacity of a Mac. And I say this at the risk of being called pedantic, but a cluster of Mac minis would have zero VRAM.