I just completed it for what I suppose must be the second time, given my familiarity with it. It was pretty good! The only parts I found hard were the white blocks at x996y1010 and the final lava screens, so I'd say you put this squarely in the Very Hard category, at least for completion of the secret ending; I don't think the normal ending really merits it, but YMMV.
What the heck is the clue at x988y996 supposed to mean? How does one
shoot for the stars? I presume by "the small Red Island" you mean x990y996.
I think the location of the secret ending might be insufficiently hinted at, though I agree that the clues are fairly good in retrospect. Perhaps you could put in a line somewhere near the end about
a good designer not including useless screens?