How to remove load_video and gfxmode from grub?
Per code in /etc/grub.d/00_header, it looks like you need to set GRUB_TERMINAL=console to avoid gfxterm
It's also possible to set GRUB_VIDEO_BACKEND to a specific value to avoid a lot of video modules, but I'm not sure what actual value is the best thing to use here to avoid video altogether, cf. https://www.gnu.org/software/grub/manual/grub/html_node/Simple-configuration.html