At how many first pitches would you start using F-strike% to make a decision on a player? 200? Let's say your guy is sitting at a 65% F-strike% after 200 hitters, which would be elite. League average is 60%. So you're talking about 10 strikes separating elite from average, possibly confounded by umpire, QoC, framing bias etc... So it's pretty dubious. And at that point you're at 50-60 IP, so the skills you're trying to tease out have already manifested themselves in a meaningful walk rate, with a K-rate approaching meaningful.
I just don't see the utility.