I am not sure your point on advanced metrics holds up. Advanced metrics preferred Mitchell, which was absolutely correct. We’re still waiting on some of the metrics to update for 2021 data, but RPM has Jaylen not insignificantly ahead in their extremely noisy first update for 2021 (Jaylen is at 4.60 rating/3.02 wins and Mitchell is at 3.41/2.34). RAPTOR currently has Jaylen as a +2.9 overall player and Mitchell as a +1.3. BPM has Jaylen at 6.6 and Mitchell at 0.7.On the other hand, Mitchell has carried an offense for multiple years, including effectively in the playoffs, and at absolute most Brown has done that for about the last month. That track record difference matters.
Advanced metrics prefer Mitchell, which is worth something (but only something).
I think you have it backwards a bit--there was not a credible, non-fanboy argument for Jaylen over Mitchell until the recent leap. Now, I think there can be and the reasons you state are reasonable. I still lean the other way, but by end of regular season this year might be on your side if things continue as they have been.