The original D7 was indeed a spectacular and inspired piece of design in its own right. MJ completely avoided something that could have looked throwaway. It's amazing really considering the ship appeared but three times and all in TOS' third season ("Elaan Of Troyius," "Day Of The Dove" and "The Enterprise Incident."). It would reappear again a few times in TAS.
It was something that managed to look menacing as well as beautiful all at the same time. I think the D7 and the TMP K'tinga version are far superior designs to the movie and TNG era Klingon BoP as well as the subsequent TNG era Klingon designs.
The original Romulan warship from "Balance Of Terror" is also a nice design, but it doesn't resonate the same way. Mind you it serves its purpose in being something that definitely looks less adanced than the Enterprise design. But it wouldn't be until TNG that the Romulans would get an inspired design of their own.
MJ was able to in one design, capture and evoke the style of an alien race in a way that few inspired designers have been able to do since. When you look at his sketches for it, it was clear from the get-go that he had a really solid idea for what KDF warships would look like. The budget afforded TMP allowed them to flesh this out in a way that made them feel even more real. I agree that the future TNG/VOY/DS9 Klingon designs felt tepid in comparison. They weren't intimidating like the D7/K'Tinga was, and they certainly got away from what made the D7 feel alien.
Except for the BoP. For me, this TSFS design is right behind the D7/K'Tinga. Despite its small size, it's every bit as intimidating and dangerous as the D7. Its morphology and details feel right, to me.

And you're also right about the lackluster Romulan design for TOS. It looks like a design made to resemble a UFP/SF design for some clandestine mission. Too bad we never saw THAT mission.
Not to bring up "Enterprise" again, but they did kind of get the Romulan BoP right, if only with the benefit of hindsight, taking the best elements from the TNG-era D'Deridex (which is the inspired starship the Romulans deserved all along), coupling them with the morphology of the TOS BoP. Is it retconning? Absolutely. Does it work? Seamlessly well.