So, the 4-bit gui seems workable. But, I wanted to try a 2-bit gui before going all in.
2-bits lets us use P1 ROM font and allows higher resolutions.
The font also looks smoother than with the 4bpp mode because the characters are 16x32 instead of 8x16.
This is just a test at VGA and XGA resolution (tested on P2V13, but think will work on P2V15 too):