It's easy to believe in a monotonous decreasing of variety in starship design, with TOS designs still exactly as diverse as the DSC ones, only lamentably unseen, and with the late 22nd century actually wilder than even a quarrelsome committee of John Eaveses could imagine.
It's equally possible that the DSC level of diversity remains in the TNG century, though. We did see oddballs in the background even before CGI debuted big time in the TNG movies. If anything, DSC gives us a steady stream of round saucers, while the other eras have a dearth of those.
Inboard engines don't seem like an alien design solution that would be on its way out - ships like Saber keep the warp engines tucked close in, even if the configuration of the Defiant looks odd even to our DS9 heroes initially. The Magee really is pretty traditional in the end, with two nacelles, a round saucer, a topside bridge, a ventral deflector, the works.
Indeed, as the above experiments show, a paint job and an engine swap do wonders here. Although I don't think TOS engines are a thing that should succeed the DSC boxes. To the contrary, the round-tipped cylinders could very well be antiques, relics of the ENT era that only remain in civilian service in the 2260s, along with the few surviving Constitutions and perhaps two or three other similarly ancient Starfleet designs.
I say, bring on more DSC ship mods with TMP nacelles to get a feel of how the 2260s really looked like, outside our narrow scope of TOSvision!
Timo Saloniemi