Not everyone with dedication or a brain for the craft. It is rare (not impossible) you find a good actor for the screen that isn't already out of the price range for a bare bones indie project, which to some extent we are all dealing with. That's one of the reasons to reinforce the concept of making sure your vision/talent is contagious, especially if your vision is a unique one. Not everyone is capable of that, but you won't know until you try a few times.
As for your casting of 1200 kids, you had to have had some kind of platform/budget/extended time to pull that off, I word this to someone who might not have that so they can work to that level. Also meeting one's "acting goals" is a very subjective point of reference that's, again, based on vision. I don't doubt that you had the right kids who played to the back row and all that, but there are certain factors with acting for the screen which are much more specific than stage.
The frame of sarcasm/insincerity (and not the "haha I'm British, look at me" sarcasm) is something that I always have to teach, even if the actor I'm working with is considered accomplished in stage/voice over/etc.
Now that's just something that speaks to my style. This guy with the really good car shots could have some other kind of emotion he likes riding which is very specific in his network as well. In that case, encouraging him to turn nothing into something could be beneficial.
Nearly everything I've done involving other actors up until this point has been with what you would consider the "wrong actors" (and you're 100% right about that). However I have never has a bad read or fell behind deadline because I have the ability to quickly convince people that they want to be on the same wavelength as me. That's a major chunk of what being a film director is all about. A lot of those actors I mentioned were working for free for their own portfolio, and another chunk of those actors weren't even actors. If I was doing stage my advice would not apply, but this guy's asking for screen advice.
End of the day takeaway; plan around your resources, then you can plan on having resources.