I looked at some pics of the filming model and indeed there are 5 rows of windows. interestingly enough,the model in Picards room, the saucer isn't as thick, nor is it on my Eaglemoss model, or even my old Micro Machine from back in the day, I'm assuming both of those models were based on the proportions of Picards model which looks to have a 2 or 3 deck saucer.
The Eaglemoss one is based on Doug Drexler's CG model, which was initially made for Stargazer novel covers. IIRC from his old Drex Files page, he did it in a rush and put it together the same way the Ready Room model was, from parts of the TMP-DE Enterprise CG model. Hence it not reflecting the proportions of the actual Stargazer filming model.
I'll be honest, aesthetically I like the Eaglemoss model, micro machine because it looks sleeker, but having those two extra decks obviously would be way more practical for a long duration explorer ship, looks be damned.
The Merian is sometimes described as 'Miranda style' and is about the same size (minimal crew crew complement though), perhaps making it more of an Oberth or Nova replacement with the 'workhorse' TBD. In terms of the 'slightly smaller workhorse', I'd say that the Luna-class is probably the best candidate vis-a-vls the Sovereign-class.
What is needed is a stop gap design that can be built quickly. Based on proven technology. I think that the Centaur would be a candidate. During the Dominion war it was an effective combatant. I imagine Centaurs being used as patrol vessels. BTW, Irish Trekkie confirmed the the Centaur's length is 210 meters. Definitely a smaller starship design.
How do we know it was effective? We only saw one and it was more of an annoyance to Sisko than a genuine threat. And obviously he was avoiding fighting back.
Well the Star Trek Official Starship Collection does have the U.S.S. Centaur, U.S.S. Curry and U.S.S. Yeager. What do the magazines say about them, perhaps they can shed some light on their origins.
According to the blurb of the Eaglemoss Centaur, this was the case. No information on the Curry is provided, save for confirmed that it is a Excelsior/Miranda hybrid, and I can't find any information on the Yeager-type as the Eaglemoss website doesn't have it any more.
The thing to remember for these ships is that they have little or no backstory to them. Almost all of the DS9 and Wolf359 ships are like this. The writers and producers asked for some different ships and Okuda's & Hutzel's teams built them, without much consideration as to their appearance or function. Almost everything I've read in these magazines were conjectural suppositions by the authors and model builders as to what they think their purposes might have been. Seeing as how the Yeager type was apparently a "one-off" (based on what some have said about it), you can infer from that that it wasn't a successful design. We can infer from the Centaur based on Sisko's admiration and familiarity with its captain that it was a better-than-average ship with a better-than-average captain and crew. Aside from that, it's all "soft canon". Lots of empty space that can easily be filled with reasonably-crafted apocrypha without stepping on any toes, but none of it would be considered gospel.
Actually, going by what we canonically saw in the show, of all the Frankenstein fleet ships, the Yeager type was actually seen the most, if we count each time stock footage of it was shown flying around the station. Which would imply that it actually wasn't a one-off (unless in-universe it was the same ship each time, but the dialogue in the show makes it clear that only the Defiant was permanently assigned to DS9.) To make the Yeager type work in my head-canon, I have to ignore the similarity of its secondary hull to an upscaled Maquis raider and just accept that it coincidentally resembles the older ship but is in fact completely different. Because attaching an Intrepid primary hull and nacelles to an upscaled Maquis raider in-universe is just stupid.
I think that, in universe, the ships resembling Enterprise Refit/Excelsior/Miranda kit bashes are old ships. Taken out of moth balls for the war. I see the Centaurs as basically Miranda-ized Excelsiors. If we accept the "test engineering hull" theory, then it is plausible that the Yeager was a one-off. (I would suppose that the test hull was built by the same outfit that built the Ju'Day/Maquis Raider). With its industrial/utilitarian appearance, I would expect that the test hull is relatively quick, easy, and cheap to build.