I'm really enjoying this. It's a big twist on the traditional way of looking at Starfleet, and I really like the vibe that the fleet is doing its best to apply cutting-edge technology to both exploration and defense, rather than the way most timelines just portray earlier ships as more primitive versions of the Enterprise.
I just want to clarify: so the original, classic Constitution design would definitely have been non-transwarp capable, right? It would have been just a bigger and better version of the Bonaventure, filling the same role of exploring the vast stretches of nearby but uncharted space? So under this, in the classic TOS era there would have been transwarp-capable flagships, but due to their horrendous cost and defense priorities they weren't being used for exploration. Excelsior was supposed to be the game changer in that it was a smaller and presumably cheaper transwarp ship, and whether it succeeded or failed, by the time of TNG transwarp on smaller ships is commonplace (I assume, based on the Defiant and Voyager. The Ent-D certainly would have been big enough to support transwarp even in this time period). Does that sound about right?