>because unlike drugs you can't realistically not use the internet ever again and nowadays it's SO easy to go down the rabbit hole even from innocent and serious sites.

I can relate so deeply to this sentiment. Especially lurking on deep web, and other pointless rabit holes.

>Every once in a while I humor myself with a building such a product, but I really don't know what features would work. Any suggestions?

Easy? Blocksite app for Android and there are many for others.