renpy is best place to start, now imagine its a rpgm instead padded with a lot of walking, or unity bloated with useless asset and cant run 15fps
Incorrect. First off, Renpy is a nice place to start but not for something like this (characters moving on screen, interactive sequences, changing bodies, you name it).
Imagine if the level of cum was visible on screen. Shooting a full load will drench the character while a small one will give a bit of drool around the mouth. And so on... Possible in Ren'Py but still only under linear/programmed limits). With another tool you could add the cum as a shader or something even on top of 2d. And it would take far less space than swapping graphics for each level.
Another con: Ren'Py is programmed using Python functions and is relatively slow to develop in, compared to more suitable tools. It has limits but is extensible, but only to a certain point. And it breaks relatively easily, especially when using plugins.
Visionaire - or a Unity Game engine like AGS or Playmaker with a dialogue tool makes it possible to develop in tree/branches or by coding which means, faster development and more time to focus on the art. Also, Unity has AI /behavior assets which can truly bring the characters to life. In Ren'Py, it's static coding with branches, meaning a lot more limited. The behavior package is about 5 MB, while the visual novel package is 2 MB. In Unity, only the necessary stuff is compiled/packaged in the project, this isn't the case in Ren'Py which is currently well antiquated.
Also, if you check my post further up, Ren'Py is actually more "bloated" than Unity. Unity does not add anything not necessary, Ren'Py HAS to do that to make the Python engine work.
A base Unity "Hello World" game with VN assets and including animation support, live2d etc. is about 64 MB. In Ren'Py, it's at least 80 MB. That is without animation support and live2d. So you're proven wrong (I can document it with executable examples!)
Finally, Unity can run 200+fps or more. Basically, it has no limit... It depends on your hardware. Ren'Py is a VN platform not designed for fast animation at all. Why even compare?
This game swaps static images in Ren'Py at about 2fps or for character animation, a bit faster. Ren'Py (Python) only supports what your video codec supports if playing a video file, and then it isn't even Ren'Py (python) doing the work, but the codec, which you need to have installed manually if the dev uses something specific.
If given enough time, I could write pages about why not to choose Ren'Py. Unity is cross compatible, which Ren'Py isn't. If you write something that works in Win/Lin/Mac, you have to make sure to do it right or it will not work on one or more platform(s). You're free from all that using Unity.
And then I even forgot the details about Visionaire which is used by numerous games in here, and which is the best option for point-and-click games beside Unity with included assets. Visionaire does something similar to the old times' Leasure Suit Larry and similar games and supports scrolling, making it possible to add larger floors in this game (there are so many things where you have to think out of the box with Ren'Py and I do promise you, it will break for some users).