Basically same as other categories such as Classic 5.0 and Classic 9.9, but after finishing Round 6 the intensity should increase to 9.9 and clear Final Round with said intensity.

That is practically what 9.9 already is, someone can do a run of both 5.0 and 9.9 at the same time. I don't entirely understand what you're asking, but it seems that it would provide inconsistency with the other categories which we would not allow.

From I have seen some runs clearly reaches 9.9 but AFTER clearing final round (boss round). What I suggest is starting final round with said intensity. But if that category provides said inconsistencies, then it would be the best to discard the idea.

The minimal difficulty gap doesn't really make much of a difference, and has no real reason to have such a specific requirement for a category.

The first runs I did of 9.9 when it first made to leaderboards were done in that way. Mainly because I just wanted to do every characters run without a death. But after more runs and finding that starting at a difficulty lower than 5.0 can save over 30 seconds of time, there is no reason to start at 5.0. While the run still follows the same format and rules. Nothing about the run really changes except the difficulty on the first few stages. Which is changed to making the end of the run much more intense because the small mistakes or time it takes to beat a stage may result in under a 9.9 run.

Making a separate category just for the final stage to be on 9.9 is arbitrary and changes hardly anything, other than about 15 seconds of doing weaker damage to the Boss.