Reducing video mode switching during Linux boot

Plymouth is designed to help in this regards:

The idea is that early on in the boot process the native mode for the computer is set, plymouth uses that mode, and that mode stays throughout the entire boot process up to and after X starts. Ideally, the goal is to get rid of all flicker during startup.

As you only have a single OS installed, disabling the grub resolution changes (no background graphic), and even the menu itself, leaving it 80x25 would result in no change until Plymouth takes over.

Probe your resolution and your colour depth:

xrandr -q
xdpyinfo | grep 'depth of root window' | awk '{ print $5 }'

Set GRUB_GFXMODE=<width>x<height>x<depth> in /etc/default/grub so that the grub menu resolution matches the KMS resolution.

Maybe add GRUB_GFXPAYLOAD_LINUX=keep to the defaults as well, so that there is no intermediate step after the grub menu and before setting up KMS. X11 should keep what KMS uses, meaning there is no X11 switch to worry about.

Apply your edits with

sudo update-grub