My idea is to make the pop-up "are you sending this post to someone" toggle-able.

I take a lot of screenshots on Reddit simply for myself and if I am sending a screenshot to someone, it's because I want them to see a very specific part of a post and don't want them to have to click a link and scroll through a bunch of text to see what I'm sharing with them. I imagine most users at this point understand that sharing a post is an option and don't need to be reminded of that.

A better solution if you want to still remind people to share posts is to make this a toggle-able feature. That way, people who don't know to share are still reminded, and those who already know and don't want to can turn it off.