I use the same MacPro with only 64Go RAM
The same OS
With two Nvidia like this... The 3rd on left side is not connected
And this is my octane "crash test"
I use AC20, 21, 22 without any problem (never crash or graphic bug)
I use C4D too without any bug
My MacPro still runs ArchiCAD quite well and it has slower processors that yours, though the last version I installed was 21. I've also got an nvme drive in my MacPro, but in all honesty for general desktop use the SATA SSD is perfectly fine.
I've also installed an RX 580 gfx card which really makes a big difference to to AC working environment. My RX 580 is currently in my linux box with the GTX 680 in my Mac Pro.
2012 13" Macbook Pro 8GB Ram, OS X 10.14.6
2010 Mac Pro 2x 6 Core 2.93Ghz Xeon, 48Gb Ram, OS X 10.14.6, RX 580
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