for most of its life, Project Diva was about very strict timing and precise button presses
Yeah, I have to say that I definitely disagree with this assessment. Granted, I never played the first game, but from 2nd onward, the COOL window's always been fairly large, and even just keeping a combo gets you a "PERFECT" rating regardless of your COOL count. I think those, combined with the overall easier difficulty compared to other music games, is enough evidence that this is a fairly casual game as far as timing.
The fact that the star notes have even less strict requirements than other notes (just swipe madly and you get to keep your combo going) mean that if you don't like them, you can just BS your way through them. Essentially, you can ignore them if you want to.
Really, though, I think the star notes serve two purposes: to take some sort of advantage of the Vita hardware, as many early games tend to do, and to add some sort of new depth to the series. Personally, I think it totally works at both of these, and having to switch between buttons and touch--sometimes on VERY short notice--adds a bit of challenge that wasn't there before.
You can't have game after game of the exact same gameplay with no changes. It'll get very stale unless it has something else. Example: IIDX hasn't changed at all, but its sheer difficulty along with the great music is what keeps gamers coming back. This game doesn't have difficulty going for it, so they need to innovate a bit...this was an attempt at that.
And in terms of the mini controller, what is the size relatable to?
The design very much reminds me of their Fighting Stick line of joysticks...which is to say, their budget line. The Real Arcade Pro series is much larger and uses real arcade-grade parts. I would expect the mini controller to be just a tad larger than a typical Hori Fighting Stick...which is why I'm not sure I think it's worth the price tag. But it is what it is...