His DER was .721, .714, and .709 in S7, S8, and S9 respectively.
I think you were a bit unlucky defensively in S9. Without checking, I would bet that most teams with an above-average fielding percentage and a +24 +/- will not underperform their FIP by 0.4. Interestingly enough, although you had a +24 +/-, your DER in S9 (.709) was worse than league-average, which was .714 for that season. That could have a lot to do with a ground-ball heavy staff though, as ground balls have higher batting averages than fly balls that stay in the yard.
I do believe that outfield +/- is more important than infield. Your infield defense was quite good, you were just terrible in left and right field. Combine that with a home park that already boosts doubles and triples, and it's probably not a great competition. As you mentioned though, it was a worthwhile trade-off, because the offensive boost easily made up for it.
Another factor is catcher defense.
In S8, you threw out runners at a .373 clip. In S9, that number was reduced to .223. In 130 attempts, that's a difference of about 20 outs. For simplicity's sake, I'll ballpark each of these outs as worth about .4 runs, and say that your catchers were 8 runs worse at throwing out runners in S9. Additionally, your "weighted pitch calling" (the pitch rating of your catchers times their share of innings they caught) dropped from 78 to 71. You've mentioned before that you think 10 points in pitch calling = .1 in ERA, so call that .07 in ERA, and that's another 9ish runs. So overall, your catcher defense could be estimated to be in the ballpark of 17 runs worse in S19, which seems to put your non-catcher defense at about 50 runs worse from S8 to S9.
So, combine a weaker defense not optimally suited for the park, with catchers significantly worse at throwing out runners and calling games, and add in a little bit of bad luck and a heavy pitcher's park (Santa Cruz) in your division being swapped for a slight hitter's park (Salt Lake) and you have a recipe for a pretty big increase in ERA.