The problem was hitting (at the end when the injuries mounted) fielding, and middle relief. in the last 6 weeks of the season they went from first in runs scored to fourth. The fielding was awful, and the fielding metrics don’t do justice to the pitching ERAs are ballooned by terrible fielding.
All the complaints about losing Gray, while it would have been awesome to have him, SP wasn’t the problem. Twins Sps were 5th in the AL in WAR, first in K rate, second in BB rate, 3rd in xFIP.
Edit to add, I think they added depth in the right places, but bad additions, exacerbated by incessant platooning