Disabling Secure Boot is not the same as enabling CSM (non-UEFI boot). You can disable Secure Boot while still booting via UEFI without CSM, and that shouldn't

Disabling Secure Boot is not the same as enabling CSM (non-UEFI boot). You can disable Secure Boot while still booting via UEFI without CSM, and that shouldn't affect your graphics at all.