Ubuntu Mate 20.04: Low Resolution In Grub and Splash (NVIDIA related?)

I'm on Ubuntu Mate 20.04, Nvidia GeForce GTX 660 (yes an oldie) with current drivers enabled (nvidia-driver-450). I'm currently on a dual monitor setup with a LG IPS FULLHD 1920x1080 (HDMI) as primary and an Asus VE247 1920x1080 (DVI-D).

I think my problem is very relatable to these followings (which solutions I've already tried with no success):


Talking about grub, my only available resolution options (shown by videoinfo inside grub command line) are:

Adapter 'EFI GOP driver'
1024x768x32 [default]

While the splash screen has also a wrong resolution (which I suppose is strongly connected to the grub issue) but that surprisingly is corrected to 1920x1080 if I enable the xserver-xorg-video-nouveau drivers (while it's still a no-go for grub resolution).

An interesting behavior is changing the integrated graphics devices configuration from BIOS. Setting the IGD mode for boot shows for the same driver more options but of course it means I should put the monitor cable (HDMI) directly onto the motherboard output, which is something I'm not looking forward to.

Adapter 'EFI GOP driver'
1920x1080x32 [default]

As I said, I've already tried a lot of solutions but with no success. For the most part I've found only old topics/answers so I bit of fresh help would be much appreciated. Thanks in advance.

UP! Can anyone help? I'd give more background informations if needed.