you're on the right track for a great little niche.
ortho, ergo & split boards usually have trouble getting legends to match, so we typically go with blank keycaps.
your all over dyesub set is what i've been looking for (though with a couple of changes).
what i think would work, would be a "designed" blank keycap set. something more interesting than one color blank keycaps but no legends and enough keys to cover most any split / ortho / ergo board.
- all over dyesub design (again, no legends anywhere)
- cherry profile preferred over oem (if possible)
- thick pbt blanks for printing (more important than cherry vs oem)
- most "ortho keycap sets" usually only cover 5x12 (at most), i would extend this to cover 5x14. there are larger ortho sets but they usually have legends and are uniform profile.