Hate to say this but sometimes, easier is not better.
Without knowing the detail of that game’s insides, it WOULD probably be relatively easy to make a game that fits neatly into its engine. However, a codebase by nature limits game design in ways that sometimes conflict with what a developer wants to do.
Ren’Py has a huge community and wide availability of free code for functionality beyond the linear narratives it excels at, plus a codebase that is open-source. Initially making a game inside a proprietary game might be easy, but if you need to break outside that framework, things will get much harder.
It’s a limitation that devs working with RPGMaker might be familiar with, whenever the unique code language and closed portions of source code get in the way.