if you can change the value then try 256mb. some have reported that they sometimes have to load an older version of coreboot then move back to the latest. seemed to fix the no graphics loading issues for them.
you need to update all parts of your efi that pertain to open core including opencanopy and openruntime drivers you can also use this if you need to update your resource folder https://github.com/acidanthera/OcBinaryData
https://www.insanelymac.com/forum/topic/339830-dell-monitoring-by-smm-and-fan-control/
there is also https://github.com/acidanthera/VirtualSMC/tree/master/Sensors/SMCDellSensors
try different values of finger z and see if it gets better or worse so if 10 is harder and 99 is faster or not if nothing changes then issue may lay in some of the other values.
what happens if you close the screen and open it again after it blacks out on the main screen? if you shine light on the screen do you still have images showing but with no backlight?
you can run the config validator to see what needs to be fixed in your config. should tell you the errors. then you look in the sample config to see how it is now implemented as things change in OC on a monthly basis. make sure all acidanthera kexts are also updated with the same version of OC that is installed. simpler to update to newest stuff since it gets hard tracking what goes with what version.