Yeah, the current engine is pretty annoying to work with and a lot of the bugs are caused by some plugins not working too well with another, I'm trying my best to fix what I can but some bugs will probably have to stay until I move the game to Unity.
About the Hololive girls, I'm a fan of them, and I know a lot of people are too.
I thought they would fit the theme of my game, so I added them, I'm glad it worked out too, from what I heard from some people.
And yes, I do have plans to add a few more girls from EN, JP and ID!
(I've already briefly mentioned Ina on Calli/Gura's quest too!)
About the combat, I should be doing a few tweaks to balance it more since I've added new bats/skills this update, but overall it should stay the same for simplicity and due to limitations of the engine.
I do plan to make a few changes to the combat on Unity.