Miranda-class time? It's got more volume than the Connie and same weapons fit, and was built in ridiculous numbers, our version would probably be similar (though perhaps with 2x forward RFL).
I personally dont like the Miranda's design. But yes, something along those lines will be necessary.
Im looking at this entire thing, and Starfleet keeps losing ships, and they were already arguably understrength for the job they were supposed to be doing.
Not to mention that due to the Warp 8 transition, most of the fleet is obsolete.
There needs to be a massive buildout up and down the fleet roster.
Else we'll be doing this again, with the Romulans or the Tholians or someone new.
The cost rating being C+ and the economy of the Federation being gutted by first the overextension and then the war. The need to have more ships to cover the far-flung spacelanes for anti-piracy patrols means that dedicated warships for fighting peer-power navies get short shrift. 18 C+ Excaliburs is already 4 more than the A+ cost Constitutions in the main timeline, and so quite a bit more total cost.
We built 12x Sagmartha-class explorers while on a lower budget; at almost 300,000 tons, they are the biggest starships we've ever built, and they cost way more than the Excaliburs.
I dont think cost is going to be the issue here; the lost colonies are not yet contributing factors to the Federation economy.
A period of existential threat focuses priorities and loosens budgets, and Starfleet's core industry remains untouched.
The geopolitical situation appears to be very different from canon, and the politicians just got a salutory lesson in what being cheap gets you.
Im just not looking at a post-War situation where the Federation has both the Tholians and the Romulans on their borders, Starfleet has lost anywhere from a quarter to half of their fleet of ships, and continue quibbling about costs.
It just doesnt seem like the usual sapient response to this kind of thing.
I mean, the Sagmarthas are obsolete, and the Newtons and Selachiis are absolutely getting murdered.
There's a lot of replacements that have to happen.