Good concept but I don't think Ren'Py is the best choice for this type of game.
If dev reads this and wants to make some really amazing sex sim game here are some tips.
Switch to unity or unreal, really any engine that allows actual animations and not just pre-rendered stuff and then make actual animations.
More girls and option to customize them, I don't mind inspiring designs on other games but player should have the ability to change size not of just yourself but also of the girls, this with proper, dynamic animations that actually react to the size would be amazing.
Cervix penetration - it is there but only for 3 girls, I'd add a parameter of penetration force and threshold for penetration (that should also change with amount of cum, more cum = easier to penetrate due to more lubrication) that way it's possible for all girls, realism aside of course one would make this penetration a sort of ultimate goal, bascially getting the girl to climax + this = better score resuliting in more points towards unlocking new positions, toys, new reactions, etc.
More positions for sure.
Give girls more character, turn it into a proper sex sim with dating/training aspects, albeit still with focus on penis size, penetration depth, amount of cum.
Aside from toys and new positions I'd also add some stuff that lets you change penis size dynamically, make it smaller or larger with use of some kind of items you can cosume before or durign sex, also for increasing amount of cum, etc.
Lastly I'd work on reactions, don't wont' to be a downer or support certain world views but the girls are way too forgiving, I mean my dick is at best average, and actually below but all girls said it was huge, which is isn't but for those that wish to not go into SPH maybe some settings to adjust what is small and large to differently sized girls.