I agree a rotation upgrade is ultimately going to be more necessary/meaningful than a bullpen upgrade, though I still don't think there's a rush. If you had to spend their $ tomorrow though,
I'd do it on Keuchel or even a Gio flier over Kimbrel (and if you do that then Pivetta may ultimately be relief fortification, as Anderson already is and De Los Santos likely will be).
To me this week was on the offense, injured thought it might be. A rotation that gives you 5-6 innings and allows 4 runs should be enough most nights in 2019 (heck, a classic "quality start" is a 4.50 ERA), even if the pen costs you one every once in a while. With our offense we should (and can) ding other bullpens too.
5-5 in the last 10, 4-5 on the road, turn that last number into 6-6 this series and that's fine. You take two guys out of this line-up though and it ripples, you notice more when the big guys faile and the 6-8 spots in the line-up become holes (especially on a day that Knapp starts, and with Quinn a non-factor for now).
The pressure on the pen would be relieved if they cut Alvarez or Nicasio, I'd have no problem seeing Rios or Ramos or Pazos getting some of those lesser innings, cycling up and down. But one's a lefty and one makes a lot of $ so they may just intend to ride it out, at least until Robertson and Arano are back (Arano will certainly be back soon).