The progress on this one is going slowly, but surely. The first game felt like it took ages to develop, but I guess that's the dev's approach and we should respect that. With all respect, both this game and the first one look amazing.
IIRC, the reason sinisistar took a long time to develop is because a lot of changes in the game mechanic like the main menu. Besides that, at first Nennai's initial plan for sinisistar is pure ryona game (with a few tentacle action but still focuses on ryona), but because of the pressure of follower on twitter at that time, Nennai did 180 degree turn from their initial plan
so, if we're lucky (no major changes or anything else), it'll took 2 more year or 3 years max (if we're really lucky, maybe 1 year), not to mention if i'm not wrong Nennai got a fulltime programmer now.
But this is only my assumption