I've been thinking about this one for a while. I actually started dividing the database into different categories, but then realized there are a bunch of words that fall into multiple categories, making it hard to ever turn off all of the terms you don't want.
Then there's the fact that while you may not be doing posts about movies (or another category), other people are or potentially will be, so deciding which categories to have off by default is also not easy.
At some point I will get some solution implemented though. Another option is to get AI to decide which links are inappropriate for your content and remove them, but that's another can of worms. AI is not good at making those kinds of decisions yet and will no doubt require constant tweaking.
You've obviously given this more thought than I have. I figured films is a good start because they all contain -film- in the url, and that movie titles often are the same as common words. In my experience they're 90% of the words I end up adjusting.
Anyways, looking forward to further updates! Totally agree with your view about the On/off by the default part