I disagree with Cilic being a step below Thiem. The numbers say otherwise. Cilic has over 600 match wins, whereas Thiem finished with 'only' 348. That's a huge gulf in between.
I may be emphasizing peak more than you. Cilic played a long time, and for much of it he was actually a half-step below Tsonga/Berdych, more of a top 20 guy. Then he had a nice late peak in 2014-18 where he was a fixture in the top 10, basically Tsonga/Berdych with one Slam where he did a Stanimal. But I don't think he was really a better player than the solid 2nd tier types. I think Thiem was overall better in his prime.
Take their best five year spans:
Cilic (2014-18): 9 titles, 1 Masters, 1 Slam; 2 Slam Finals, 1 SF. 225-105 (68.18%)
Thiem (2016-20): 14 titles, 1 Masters, 1 Slam; 3 Slam Finals, 2 SF. 234-99 (70.27%).
It is close, but Thiem has the edge. Now overall, I think they're comparable. Cilic gets the edge for longevity, Thiem for prime.