Stuck with bootresolution being too low

Had this problem just recently.

Have you tried:

When the boot menu appears:

Pressing one of the arrow keys to stop the countdown on the boot menu.

Then press the r key repeatedly to cycle through the available resolutions.

Once you have found the resolution you want just choose your kernel to boot. Next boot your choice of resolution will be remembered.

You can set your preferred kernel by moving to it and pressing d.

You will probably have to set it again if there is an update to systemd.

Hope this helps.