Honestly, if this is going where I think it's going, I can see this easily being done in the time frame MITY said they were aiming for.
I imagine that book 5 is going to be a lot less content heavy and mainly just tie up the plot. Like the player had to go across time seeding people with the Avatar's energy for some spiritual reason or something. When all the spirit stuff is resolved, the protag gets their big Valhalla ending. A spirit world harem with all of the girls you met across time. And this would basically just be an in game way to replay all of the sex scenes, no gallery mod needed, and maybe some new scenes as well.
I'm of course just spitballing speculations here. Whatever happens, and however it ends, it's been a hell of a ride.