I don't understand why I can't call Succubus Contract II a sequel?
It is an improved game in every aspect.
The first part ends on intrigue, the second part continues it.
For something to be a sequel, the thing it is a sequel of has to be a fully contained story. There are many other games here on F95 that do it in chapters as well, there is nothing wrong with that and you shouldn't be ashamed to call what you've made Chapter 1 and Chapter 2
Future chapters will always be better as you have gained more experience so the longer you keep making renpys and renders the better your work will become, but simply because chapter 2 is much improved over chapter 1 doesn't make it a sequel
A sequel is defined by the story and story wise Succubus Contract (Chapter) I is not a finished story, it just stops and then you continue it in Succubus Contract (Chapter) II
But regardless of what you call it, I am extremely hyped to just see Succubus Contract continued in II, love your work, love your renders and love the story. Also great how the choices do feel like they have an impact and you can see vastly different renders based on your choices
Anyway that aside, with the release of Succubus Contract II, does this mean that Succubus Contract I is now complete and will receive no further updates?