How cheap (But not poorly) could you build a "Mac Pro?"

This is more of a fun question rather than a request for info. :) But still would love to hear about working/proven builds.
Everything has to work (Bluetooth, Handoff, Wifi, Airdrop, and all software), and needs TB3, and preferably 1-3 spare PCI-E slots for future expansion.

And if there's any major compromises made to make this work, what are they?

Looking for 3 Options:
Base Configuration:
https://www.apple.com/au/shop/buy-mac/mac-pro/tower# (This is the Aussie website/prices) ($9999au)
https://www.apple.com/shop/buy-mac/mac-pro/tower# ( This is the US website/prices) ($5999us - which is over $1000 by the way, after exchange rate).
Custom (Can use different parts and vary slightly):
- 16 Core (3.2GHZ/4.4Boost CPU),
- 128gb RAM (Expandable to 256gb or more) (Prefer big memory in less slots, within reason)
- Radeon VII or Radeon Pro Vega II Duo GPU (Unless you have a better idea - it's a workstation with some gaming).
- 2TB SSD Storage (4TB Preferred)
- A Big internal HDD for storage.(Optional).

Fantasy Option:
As above, but 256gb RAM Standard (with extra expandability), dual GPU's as above, and 24-28 cores.

Prefer No RGB Gaming lights etc. Simple, and elegant.