I think I've got the game figured out for the most part after messing with things more on my own and reading some tips over at GameFAQs.
Here's my mini review so far. There is no gameplay in terms of real action or interactive mini-games. The "game" here is setting up your stats, leveling up the girls and swimsuits, and battling it out in volleyball. You don't play volleyball, though. It's automatic and the outcome is determined by stats. You do get to pick strategies and swap out characters during the match, so there is some interaction that you can use to your advantage. Outside of that, it's pretty much what you'd expect from any DOAX game; a dress-up, picture taking, gambling simulator.
There are 100 gravure scenes. I've unlocked a few for Momiji and Misaki. So far the scenes I have are recycled from DOAX3. Misaki has a few of Kasumi's, and Momiji has some of her old ones plus the latest one I unlocked which was Nyotengu's old DOAX3 hammock pose (wink, wink). I'm guessing there are some new gravures based on the trailers, but I haven't unlocked any yet.
Is this a game I would spend money on? From what I can tell so far, no. But it's free and the micro-transactions only seem to be a boost rather than exclusive content, though that could very well change. For now, I enjoy it for what it is.