What happened with PhenixTool - did you follow the procedure like above and it all went well with no errors at all - but then when you flash it did not work? I never needed to use flash-back to comment on it - but always used the standard flash (EzBIOS Flash) in the BIOS itself. I use Intel platform (not AMD), so I only used Asus Z390, Z590, X299 (old BIOS thus original PhoenixTool) and new Asus B660, B760 (new BIOS thus modified PhoenixTool) but they have all worked just fine. I've had issues modifying some BIOSes but once they did build flashing them has always worked. I think the idea is to always upgrade to the latest original BIOS if not already on that then flash over it the SLIC-modified BIOS of the same version through the normal BIOS EzFlash. This way it should only add the new module with the SLIC blob and not anything else.
I followed instructions above & SLIC was built successfully but did not take. So then went to flashback. Maybe because I have an Asus ROG CROSSHAIR X670E EXTREME board. Great now I have to haul her down to the local shop. You win some you loose some.
That's a pity. Perhaps another option would be to run a KMS server in a VM somewhere on your network and activate using it? If you set up a "_kms" forwarder in AD/DNS to it then it will activate automatically.
I encountered a similar issue where PhoenixTool does not work on MSI H110M PRO-V, but works on HM77 and GA-B85M-D2V (rev. 3.0).
Check out the "fix" above (checking for and modifying a different GUID) - so far it has worked on the new ASUS BIOSes that I've tried.