Vega 64 not displaying since Mojave 10.14.6 update

Jun 9, 2020
Mac Pro 5,1
12 Core Intel Xeon
AMD Vega 64
Mac Pro
Hi all
New user here so sorry for my ignorance (and if there is a more suitable thread please redirect me).

I've got a Mac Pr0 5,1 12 core 3.06GHz (6 Core Xeon X5675 x2) with 64gb ram and an ICY dock drive with 2x512gb Samsung 850 ssd drives in raid (total ssd overkill i know but i was young and stupid when i put the order together!!).

I recently changed from a Nvidia 980ti flashed for mac from macvidcards to a Saphire Radeon Vega 64 8gb card with great success using the Pixlas mod. Everything was fine until I updated to 10.14.6 and now I have a black screen when starting up. Nothing happens.

I have installed 10.13 to a spare drive and tested it which seems to work fine. I haven't stress tested it, just wanted to see if the graphics card and system was still working ok as is (as the gpu that came with is not Metal supported, the ATI Radeon HD 5770 I believe, so couldn't use that to test the machine or the drive).
I've been trying to install an earlier version than 10.14.6 but I can't find an install package that isn't direct from Apple and is 14.6 or is 14.0.022 and the certificates are expired and won't let me install it from a usb installed boot drive.

Does anyone have any suggestions?

Just to say I am not a mac hardware hacker in an great respect and have limited knowledge and skill but I'll try anything!
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