How flat??
According to the data I have from IM Maryland (which is pretty much as flat as it gets!) I spent 85% of the race either in the 55x 14, 15 and 16 and the sum total of just over 2 minutes in any of the 11 and 12, and the 25, 23 and 21.
If I were looking to change cassettes for a flat race like that again I’d try to make a franken block, a straight through 12-23. Obviously it depends on your strengths and exactly how flat the race is but especially as your on compact gearing at the front, I’m with @cpooleNZ that an 11-23 should work fine.