whenever you hit a D only with one red key, you get a forced 100 (without the x4 bonus) and break combo like a sliderbreak, but you don't miss. This still accounts for actually hitting something, but seem to punish you with sliderbreaking and ignoring the double hit mechanic. I like that more than what it is now, tbh and I will try to bring that forward instead.combobreak vs miss on a finisher from not using both keys... correct me if i'm wrong but does this change anything apart from giving you an S rank instead of an A assuming FC on the rest of this map? because if not, then it's doing nothing meaningful to the score apart from the extra points you got from getting a 100 vs a miss, which is negligible.
if anything it should be more like missing a slider end in standard, but since taiko isn't combo-based like standard is you can't really draw that comparison. i still think a score drop is enough, even a more harsh score drop as you are implying, but i don't think you should break combo from missing a finisher.