Reno and Tilly don't count as "legacy characters" as they were introduced in one of the modern shows. The term "legacy era" only applies to the shows and movies from 1966-2005.
Never heard of it termed as such.
As it is, just going from Disco, Admiral Vance was already a bit too hands-on for Starfleet CinC. SFA is just continuing the precedent established there. Which to be fair, is a common trope in fiction whenever we see the head of an organization like this on a regular or recurring basis, they end up becoming a lot more hands-on than someone in their position should be.
To be fair, Starfleet was much much smaller than previously seen, so Vance is likely as active as a Fleet Admiral would have been c. TNG/DS9.