Here’s a good reference on what GPU’s support Metal. While this is aimed at PCIe boards for the older Mac Pro’s it also gives you a hint on what MXM boards of the same series. Most MXM boards have been discontented as the industry has moved on. You’ll need to do some deep searching! How Do You Find Mojave Compatible Graphics Cards For Cheese-grater Apple Mac Pro 5,1 Computers?
How about not altering your system and just run DosDude’s patcher macOS Mojave Patcher Tool for Unsupported Macs
This is a good solution for someone who is not pushing their system hard. Basically doing web surfing, email and simple writing and graphics. If you are a online gamer or heavy graphics (photo & video editing or deep layer drawing) then you’ll likely push your system to hard.
I do recommend getting a good thermal monitoring app like TG Pro which can slo help push the cooling harder when needed.