Setting grub menu resolution

I’m running 24.04 on a box with only a VGA output. I have a KVM I’m trying to use to remotely monitor/access when console view is required. For whatever reason, the capture card isn’t getting a signal unless I explicitly set the resolution. A regular monitor attached to the box has no issue, so I’m guessing there’s no negotiation happening to establish a display output resolution automatically. I end up only seeing the BIOS’s splash and text screens, and then nothing once grub’s menu would appear. I’ve managed to get truenas to set an explicit resolution via midclt call system.advanced.update '{ "kernel_extra_options": "video=640x480-32" }' and once that option is loaded ~19seconds into boot I finally get output the capture card can use.

What I haven’t figured out is how to set grub itself to use an explicit resolution. Without, I can’t make selections should I need to boot into a different environment without putting the machine out of the rack, connecting a monitor, and administering hands-on.

I’ve tried creating and placing the usually-working lines in /etc/default/grub, and while /boot/grub/grub.cfg mentions that it’s automatically generated using the /etc/grub.d/files and /etc/default/grub file, it isn’t actually putting any of the GRUB_GFXMODE= or similar lines in the final config file after running update-grub and/or grub-mkconfig. How or where can I explicitly set the resolution for the grub menu to use?

/usr/share/grub/default/grub exists, and has the proper commented line for GRUB_GFXMODE=640x480, but it’s read-only.