GRUB 2 Getting error invalid video mode specification '1680x1050x24' before boot menu, trying to increase font size

[Bob Currey]

I have a Dell Inspiron XPS 15 7559 Laptop which has very high resolution 
3840x2160 graphics for it's dual nVidia/Intel Optimus graphics, and so by 
default the characters on the boot screen are very very small and difficult to 
read.   Win/10 would boot just fine, but living with it is another thing, and 
that's why I've installed Linux and Grub.  It was tricky to get Linux to load 
with a dual boot to Win/10 due to the EFI setup, as well, but Debian 9.0 to 
both runs well and install GRUB 2, and it is controlling the boot to Debian, 
other linux distros, and Win/10, but I would really like to make the boot 
screen readable without a magnifier if I could.
I have tried editing /etc/default/grub to try various combinations of 
parameters to try to change the font or video mode but none seem to have any 
effect, and the best result I get is an error before the boot menu.  I can tell 
my changes are taking effect after I run update-grub and reboot because I have 
changed it to play an unusual tune at boot.
I ran the videoinfo command from the grub command prompt to list the video 
modes available and it says 800x600x32 is valid.
I tried creating a large unicode font, but it didn't do anythingI tried 
changing to 800x600x32 resolution, but it didn't do anythingI tried changing to 
auto resolution, but it won't accept that when I try to run update-grub
Can someone suggest a way to make the characters on the screen larger and more 
readable, please?  Maybe I'm doing something wrong?  Any help would be 
appreciated.  If you could suggest things I could try or where to find and 
example that might work, that would be great.

Snippets of things tried in /etc/default/grub follow:

#Trying a larger font I created
#GRUB_FONT=/boot/grub/fonts/unicode.pf2

GRUB_CMDLINE_LINUX_DEFAULT="quiet"
GRUB_CMDLINE_LINUX="\"acpi_osi=!Windows 2015\""

# Uncomment to disable graphical terminal (grub-pc only)
#GRUB_TERMINAL=console

# The resolution used on graphical terminal
# note that you can use only modes which your graphic card supports via VBE
# you can see them in real GRUB with the command `vbeinfo'
#GRUB_GFXMODE=autoGRUB_GFXMODE=800x600x32GRUB_GFXPAYLOAD_LINUX=keep

# Uncomment to get a beep at grub start, the tune does work when uncommented, 
so i know my changes are being tried
#GRUB_INIT_TUNE="480 440 1"
#GRUB_INIT_TUNE="312 262 3 247 3 262 3 220 3 247 3 196 3 220 3 220 3 262 3 262 
3"