The first thing is a fun idea, but tough to implement since the 'lore' is that nobody notices the bimbofication except for those that have already been bimbofied. That way we don't have to write a thousand variants of the same dialogue.

Putting h-scenes at the end of s-links is a good idea, but could have tone whiplash issues based on the ending you get. See:

Bimbo Good Ending - They're happy and sex obsessed, they'll fuck you

Bimbo Bad Ending - They're sad and sex obsessed, they'll fuck you

Non-Bimbo Good Ending - They're happy and grateful, they'll fuck you

Non-Bimbo Bad Ending - They're sad, they might fuck you because they hope it might cheer them up?

Having bimbo level determine how early h-scenes trigger is a good idea though, and since bimbo scales 1-10 it's easy enough to implement.

So the sex scene math could be like: 

if (bimbo level)+(conversations you've had so far)>10 {sex scene is available}

This would let you access a sex scene either with a non-bimbo with a completed story, a bimbo you've barely spoken to, or somewhere in the middle. Maybe the art for the sex scene is the same no matter what, but you gain additional dialogue if you've completed their story?