I was going to point you back here, but your dell bios update structure is slightly different than that one (and the XPS 15 9570/Precision 5530). Instead of being sure about it, I think you’re going to save the first two EfiFirmwareFileSystem2Guid volumes (in the extracted 1 Precision_5520_1.40.0 – 1 System BIOS with BIOS Guard v1.40.0 bin) “as is” with UEFITool NE, and then replace what’s in your full bios with them using UEFITool_028.
Maybe lfb6 will hop in here to advise. The more intense procedure will be cleaning the ME.
When you read the winbond bios chip, did you unplug the battery and hit the power button for good measure? Glad you got it to boot.
Pretty sure there’s a cpu microcode update you can easily apply with UBU - check cpu906E9_plat2A_ver000000FA_2024-03-07_PRD_E8FA415B.
Good luck.