I think you're right that opportunity cost is high enough that it might not happen, but I do think two starting pitcher is both #1 and #2 on the priority list. And this is coming from someone who is still *very* high on Brown. But as we're obviously seeing the team is not a lot of misfortune removed from giving starts to Chris Flexen, and that's before the load management conversations around Horton/Brown/Boyd really come to a head.
Two SPs also has the benefit of potentially impacting the bullpen depth. Giving Horton 3 more starts and then downshifting him to short relief is probably a great two birds/one stone move for managing his innings (he'd probably end the year right around 120?) and adding an arm with some oomph to the late inning mix. That's not to say if you can add two SPs it's safe to ignore the bullpen. But if you add two SPs you can shop in the Kyle Finnegan section of the store rather than the David Bednar aisle.
On the position player side, I hope the Twins start fading again because Willi Castro is a glove perfect fit for this roster. I'm not especially worried about Shaw (there's a healthy amount of BABIP regression coming his way), but you can't not address 3B at all. Castro would provide some 3B production, but also provide some support everywhere in case of injury. Like god forbid a middle infielder especially gets hurt right now.