None of this works for me on my MacPro mid 2012,5 - says "invalid command" for all the above steps I pasted in. I don't know how to find out the first step "Update High Sierra till lates 17G14042". Where is the 17G14042 displayed? I have the very latest available on High Sierra when I run the Software Updates.
Can someone please provide unambiguous step by step instructions that someone like me, who is not a PhD in computer programming can follow? Ideally, links to any external sites needed to get the software. I can't run the 387.10.10.10.40.140 installer to reinstall the drivers any longer because of this error, and the display is unusable without graphics acceleration.
I have also a while ago purchased a SAPPHIRE Radeon PULSE RX 580 8GB GDDR5 recommended by Apple for metal at
https://support.apple.com/en-us/HT208898 a while ago, but this doesn't work at all on High Sierra, and doesn't correctly align with my display's edges in Mojave, and there is no cursor.