Surely the sensible thing here would have been for them to just use starship secondary hulls if they were having to rush ships into production? Worf tells us in TNG: "Heart of Glory" that "when relieved of its bulk, the Enterprise becomes an exceptional weapon", and a Galaxy-class stardrive section is half the volume of its saucer. Also – that whole "saucer separation" thing would have been really useful in a sublight battle. We see how useful it is in TNG: "The Best of Both Worlds". Any starship that could separate has "multi-vector assault mode" built in already. Minimal refits to ensure each section has photon torpedos and impulse engines makes MUCH more sense to me than the Starfleet Corps of Engineers attempting to Lego various starships together from random parts.
While I agree with you in theory, we have no idea if the Excelsior class had a battle bridge or other auxiliary control in the secondary hull for it to be able to operate independently of the saucer. That many leftovers? And they all look exactly alike? I think we’ve gone from ‘hastily -slapped-together parts’ to ‘full-on class.’
The Enterprise-B MSD as seen in Generations does have a battle bridge labelled in the secondary hull (it's right at the front of the secondary hull's topmost deck, immediately below the neck). We also know from TNG that Main Engineering can be used as a command centre if one of the Enterprise-D's bridges is not available; it seems sensible that other ships have this capability too. But surely it'd be straightforward enough to add a temporary control centre anyway? As an example, my personal headcanon is that the unusual "server room" bridge used for the USS Sutherland in TNG: "Redemption" is because the ship's main bridge was either not installed or not functional. (We know she was "in spacedock for repairs" and Picard insisted on her being deployed to his blockade fleet "whether the yard superintendent says they're ready or not".) Edited for spelling and grammar.
FASA's Galaxy deck plans had auxiliary control being more like an extra bridge configuration, so I could see this being a fairly standard feature on many Federation ships.
Somebody mentioned that the Curry appeared designed to land on water. Really, if you replaced the saucer with wings you would have a flying boat design. Probably the largest starship that can land on a planet.
Regarding the U.S.S. Centaur.... My own idea was that it had been designed as a Miranda style derivative of the Excelsior. The prototype showed promise. But it was decided that just building more Mirandas would be cheaper than building a lot of Centaurs. What we saw on screen had been taken out of mothballs.
Perhaps they were never mothballed, but production runs of the Centaur-type were limited because Miranda-class ships just ended up lasting forever. "We have a new design for when the Mirandas need replacing! ...Any year now. Aaaaany year. Soon. Real soon. Well, soonish. Maybe." The irony being that by the time the Mirandas finally became EOL and started to need replacement they were decades old and the Centaur-type itself had also become obsolete, so it never got its moment in the sun. Maybe this also explains why we see so many Excelsiors but hardly any Ambassadors...
The Centaur we saw on screen had all those greebles on it that looked cool in action shots…but as a model just to look at…ehh. A good in-universe explanation? Prototype/retrofit tech being tried out during the Dominion War that later was made internal in newer ships.
I made it an early-2320s escort with the class name of Lirpa: Diametrically opposite to Royal Sovereign was Lirpa, a small fast escort to support expansion into and colonization of new unclaimed space. A mere 210 meters in length, half of which were the long reedy twin nacelles which stood out wide on thin outswept pylons, it was one of the smallest combat-ready ships in the fleet. The ship proper was a mere four-deck saucer with a rudimentary engineering and weapons pod attached at ventral aft from which the pylons extended. Lirpa’s small size and sparse design was a consequence of its mission parameters, which often involved convoy duties, colonization escort, and in-system patrols. Those sector patrol duties often meant they were forward-based at starbases, a necessity due to its size: the ships carried a small crew and limited space for consumables, meaning they could only operate for no more than three months on their own before requiring resupply. Sensor systems and other machinery that did not require excessive supervision or maintenance were housed in external blisters and emplacements similar to those used on Constellation to maximize habitable volume and equipment spaces. The class also mounted a scaled down yet much improved version of the conformal deflector array as first seen on the Maccabee-class tankers. The ten Type VIII phasers and four torpedo tubes Lirpa mounted gave the class quite a punch; carried within the weapons pod, the warp reactor assembly sat underneath and between the latter, providing speeds up to Warp 9.42 (1815 c).
I think Shik is describing a Perimeter Action ship. I believe that the Akyazi class is an example of this. Yeah, maybe some Centaurs were built in the early 24th century. Given a refit, enough so a Centaur can challenge a Dominion bug ship I have a hard time thinking of Centaurs as Frankenstein ships, because the aesthetics were too good. (Now, the Yeagar definitely looks like a ship slapped together). BTW what does EOL stand for? With the fleet depleted, I would expect the surviving Centaurs and Excelsiors remaining in service in the immediate post war years. As for Mirandas, I like Vales idea that these were turned into armed drones. Post war, I can image the few surviving Miranda still in use, like the DC-3 in the early 21st century.