Hey mate! Did you tried switching GPUs in your system? I suspect your theory regarding the reference card (anniversary) and Sapphire one could be correct. Actually, on Apple site regarding 3rd party GPU support, is stated “AMD Cards” for Navi and this could be important since for Polaris is otherwise recommended Sapphire. Hope to hear from you soon.
Things sometime have no straight forward explanation but perseverance and great people from this community can get you there.
What I did was:
1. I re-took the bios patching process, then flashed.
2. I copied the EFI from 10980XE system 1 to System 2. Straight away I got read of 90% of the lagging after boot.
3. I then updated to 10.15.5 which made things sweet with this EFI. Is booting faster, comparing with my 2019 MBP the boot seems almost 1:1 as fast.
4.
djlild7hina also kindly helped with the USB ssdt implementation for my motherboard.
Now both systems work great i would say. I heavy tested the both system's performance for the main use:
Premire Pro, Davinci using Arri Alexa Footage & R3D 4K, 6K & 8K Raw media. The system performs very well despite low Geekbench5 scores on Metal for 5700XT. I don't think they are so relevant to be honest in real life performance.
Flashing the Titan Ridge made impossible things possible now, patching to FM50 now the thunderbolt can read even eGPU's via Razer Core, T1&T2 adapted with the apple adaptor to T3, Thunderbolt network etc. CaseySJ guide
here.
Now sure, if I overclock the cpu's they both get much higher points in cinebench but I see no point risking with that overheating. Now they seem stable and not going over 58-60 °C in full load. Also there is a post about a way to boost the GPU's scores guide
here posted by
CaseySJ but it seems a bit complicated for now. I will stick with what the machine can do and call it a wrap for now.
Some pictures attached of both systems.
I want to shout a big thanks

to
djlild7hina who's been so patient with me and guiding trough out. I also want to thank
CaseySJ for his his incredible dedication in researching and supporting the hackintoch community.

PS: in case anyone has this config as myself, please avoid the struggle and use this EFi. You only have to add your new ID in PlatformInfo. If you unfold that section you will see the marked with "change here'.