k2pdfopt actually has an option for my model, cropping some of the excess margins helps a lot, but I will say epubcheck found some invalid markup in there, so it could be related to that.
I have updated the EPUB on Gumroad. Would you mind checking it again? I might add a PDF version with bigger a font size if it's not working.
I tried and it still doesn't work. File shows 1/1 pages, which is blank.
Added a cropped PDF with a larger font size. Hope that will work.