Haha! Well yes there were many types of Starfleet ships that participated in the dominion war including at least one Ambassador class (The Exeter). Hell, even the "Rutledge" was in the war at some point lol. The fact these many classes described weren't onscreen is simply due to the studio not having any working models CGI or otherwise at the time. It's really amazing we saw as many classes as we did and while the Ambassador class may have been less prevalent in theory I assume there were probably others. The Valdemar was on the edge of Cardassian space so I doubt it took a snooze while 10 fleets are starships were busy battling out possibly the greatest war Starfleet had faced at that point.
Regarding the two (or more) variants being built concurrently while I agree that is definitely true for Miranda's, Nebula's and Excelsior's it's really a question of why the studio decided to change the model in the first place. I can only assume the model makers sat down and said "hey let's make this thing look a bit more up to date to the TNG era and add some details." That would imply an "in universe" explanation for a refit to upgrade the old ships. Or at the very least the ones built later (20,000's) had the upgrades from the start since they were newer/built later on. While I agree with your idea Mark Nguyen
that it is similar to having two car variants and I think that explanation sounds most logical, the creators of Star Trek did not always do things that made the most sense.