I finished the game today and when I counted correctly, it has 5 chapters, so where is the rest of the game?

I remember such high flying advertisements on kickstarter of a 10 chapter game.

Will it become a DLC to be purchased separately?

It also gives the impression of an unfinished product. Everything after the last waifu's recruitment is glued together with some enhanced story-board cutscenes.

I would like to know, too

Don't expect any reaction from Studio FOW, pal. From my impression this turned out to be a sell-n-run operation.

Some missions still locked, no FOW Chan, some clothing options advertised in the DevDiarys have not made it into the game. The early production scene with the captain and Taron in the F3niX never made it into the final product.

There is a chance that they will finalize their work when this cooks up on STEAM but not on GOG.