Need help installing macOS Big Sur on Dell Precision M3800 — OpenCore EFI ready, but stuck

Here’s my situation. I prepared my EFI using OpenCore, then formatted my USB using BalenaEtcher and placed the EFI on it with minitool partition wizard.

With this setup, Ventura installs fine. But the problem is — I didn’t know beforehand that my graphics card wouldn’t work on Ventura. So now I’m trying to install Big Sur instead, and that’s not working.

I’ve also tried using a pre-made EFI for the M3800 from GitHub, but that gets stuck too — it shows the Apple logo with the progress bar, and then a stop/caution sign (the prohibitory “no entry” symbol) appears.

I need help getting Big Sur installed on my Dell Precision M3800.

Any guidance would be appreciated. I can share my config.plist and more details if needed. Thanks!