If your workflow is macOS dependent, then your likely better off doing the logic board upgrade. However if you can use Windows, and you don't have to pay to buy the software you need, you'd be a lot better off with a 16 core AMD Threadripper system.
$2000 CAD will give you a real good start on a modern system, that will flat out run circles around the best Mac Pro you are ever going to be able to build with 5+ year old tech.
https://www.newegg.ca/Product/ComboBundleDetails.aspx?ItemList=Combo.3621272
Adding a good power supply, case, ram, GPU's and drives is going to set you back some coin, over your budget, but maybe you can vulture some GPU's and drives from the system you use now.
Really, it's a no brainer, why invest in 5 year old tech that isn't going to come with any sort of meaningful warranty or support?
Spend a little more and get, what is likely the best price to performance system you can build, even if it means saving for a little longer and getting by with what you have until you can afford a few extra $$$ on a system that has some legs to carry you into the next decade, with a warranty and support for the parts you are putting out your hard earned money for.
I spent $1200 on a 8 core 16 thread Rayzen 7 1700, with 32 Gigs of DDR4 ram, a Asus B350-F mainboard, 256 GB NVME SSD, and AMD RX 580, and I got to tell you this thing is a beast. I can run the macOS on it, but I don't recommend that for a production setting.