THB maybe you can try to switch to the RestrictEvents kext, as stated on the start of the page it is the preferred method in the meantime. I switched my rig from manually mapped memory to RestrictEvents a while ago and it works fine.
So your first action point would be to create a bootable USB with OpenCore, change the config to remove the manual memory map, add RestrictEvents instead and see if you are fine booting from that USB. If everything works as expected, you could replace the config on your main drive with the updated one. Then install the additional memory and it should work right away.