We don't have any on screen evidence that it would be any ship other than the Enterprise-E. Given the Ent-D was the Federation flagship and the crew were given a new ship just 1-2 years later, I think it's fair to speculate that the Enterprise-E became the next Federation flagship.
There's also no
positive evidence that it was, as it was never referred to as such and was specifically per dialogue excluded from the post-Dominion War negotiations (and indeed evidence that it was even
part of the War is semi-canonical at best).
OTOH, several Galaxy-class starships, including the
Venture did serve during the War and in a flagship role in the ordinary sense.
It's also possible a ship filled the role briefly between Enterprises, I'll agree with you on this notion.
Why take it away from it just to reassign it to
Enterprise. I mean,
we see the
Enterprise lineage as special because those are the stories that
we've seen, but other ships/lineages could have easily been the "Hero of Another Story".
Why rename the Titan-A to Ent-G and let some other ship be the new flagship?
Why not? There have almost certainly been Federation flagships
not called
Enterprise in the past. FWIW some official social media suggests that a late 23rd/early 24th Century predecessor (NCC-1777,
Shangri-La-class) served as Federation flagship while the
Enterprise-B (and
Excelsior who I've always thought was a reasonable candidate) was in service.
Renaming the Titan was the stupidest part of the finale and reeks of bad fan-fiction.
More "bad fan service" rather than "bad fanfiction" IMO, but I agree
Admiral Beverly C. Crusher (nee Howard), MD. At last report was serving as Head of Starfleet Medical* and heading up the De-Borgification/Counter-Changeling Task Force.
* Either the Head of the Medical Branch as a whole (A post typically held by a
senior flag officer IRL as they manage 100-1000s of physicans, and known as the
Surgeon-General, Starfleet certainly doesn't have
less and Starfleet
did apparently have an SG during
TOS) or merely the Head of the
location known by the shorthand of "Starfleet Medical" (in full the Starfleet Medical Annex - San Francisco), which would be much more inline with her
displayed rank of Commodore (and indeed her having held that post as a Commander or Acting Captain in the past).