DEC750 Questions

[pfry]

[...]I wouldn't update the bios nor install a microcode plugin[...]

I concur, if consensus matters. Although I do update my BIOS, as I use consumer motherboards.

[...]3. Is there any value in enabling HyperThreading? AMD CBS -> Zen Common Options -> Core/Thread Enablement -> SMTEN[...]

It depends on your workload. SMT is generally not detrimental. You could always test it, although conclusive results might be hard to come by.
I leave it enabled on mine, but I use overpowered consumer hardware.