Okay, so by your list he is ranked 14th. I don't have playoff data and it isn't clear to me what plays they are filtering out. On all plays having passes greater than 9 yards, I have him at 15th with a passer rating of 95.7. I actually have him at 14th on 3rd down as well with a passer rating of 92.1, so not really a drop off passer rating wise.
After Cooper joins the team his passer rating jumps to 2nd in the league at 117.3 on passes greater than 9 yards on 3rd down, though we still ranked 23rd on 3rd down conversion on 3rd and 8+ yards even with Cooper.
We'll see. I keep hoping we started to see him turn the corner at the end of last year. Even in the playoff loss his QBR was 80+ -- hard to be upset about that especially after giving up 250+ yards on the ground. I just worry about the rush to pay him, but hopefully we look back and say it was a bargain.
I'm okay with Dak. We could do a lot worse, and have in the past. Just get annoyed by all the Dak is elite threads because it isn't an elite passer, at least not yet.
P.S. all the data is available from 2009 on so if you know R at all you could be looking at this data yourself.
https://www.rdocumentation.org/packages/nflscrapR/versions/1.4.0