As for the animation, I would like to see if I can put this up on Steam at some point. With some hesitation I've concluded the game would be more presentable if it's advertised that there's no penetration. I swear it sounded smarter in my head...
Also, it cheapens the animation if it get reused later.
to be honest - in the original "birth game" - one of the hottest parts was the "rounds" on which fertilization took place (In those where it was)
It's even rather simple to implement, without much branching of the plot or animations.
For example, just slightly expand the first "throws" of the game. First throw, for girls to overcome the interval of 5-7 cells to the "ovulation" field segment. And the second move, in which they all "accidentally" end up somewhere on the strip "fertilization" (which are second after the previous one).
After everyone completes their moves - they all grab their tummies or groin at once with "frightened" faces, feeling something slide inside the aaaaand- ...boom magic happened.
(there is already such an animation in main game if I remember correctly. + the initial stage of masturbation animation is also suitable)
(for "additional cards" in these rounds, you can take something harmless, without much impact on the later stages of the game. For example, some "double creampie" card, used on MC or something)
Then you can add a short dialogue where they are desperately blushing trying to come up with a "rational explanation" for what just happened.
And actually that's all. then you can continue the game as it already is, with existing branches and other things.
Just thoughts