I can't say I agree with anything you said here.TMP was in line to the ultra preachy season 3 of TOS, but many fans had or dismissed that season and looked upon how awesome and cool the previous seasons were "Action and Adventure". Probably wasn't the kind of vehicle to start a series of pictures, but thank goodness TWOK did that, but the series of movies were a mixed bag. Just imagine the possibilities if Harve Bennett didn't hire Leonard Nimoy to direct Star Trek III, and stuck with his guns in developing Saavik and the plot was Romulan related? The possibilities. Star Trek II has so much going for it, while Star Trek III napalmed the direction of epic ideas.
Being "preachy" was never TOS' problem. The problems were when it was poorly executed or driven by an asinine plot, further hampered by budgets requiring heavy corner cutting, which were the biggest problems with season 3. TMP's failing was that it was built off a script intended for television, not film. The two don't exactly jive oftentimes. Especially when you have a story intended to introduce new characters that would persist through a series,... but now you're making a film that uses them as a one-off, so what would have worked as their introduction only serves to overshadow your actual main cast. Decker and Ilia's scenes would have been seen as truly touching, if we followed a series that showed the progression of their characters afterward.
"Action and Adventure" weren't the biggest selling points of Star Trek, and many of its most well-regarded episodes were instead about drama or even comedy. Case in point, "City on the Edge of Forever" and "Trouble with Tribbles"
Wrath of Khan basically served to make Star Trek accessible to the general audience. A story easier for the layman to understand, with plenty of backstory provided to fill in the gaps, and a cool space battle to give the spectacle-hungry something to enjoy. Sure, it worked great as a film... but you mention "epic ideas"... what epic idea was in Wrath of Khan aside from Genesis? And in that film, it was used as little more than a bargaining chip.
Wrath of Khan focused almost exclusively on Kirk and Spock, with some Saavik thrown in there. Everyone else (certainly the original TOS cast) was relegated to background roles. Even McCoy was almost an afterthought. You say Wrath of Khan had so much going for it,... in that I agree... up until the point where Khan was obsessed with revenge. Then virtually everything compelling, threatening, and menacing about his character was stripped away. He was no longer manipulative with his charm and intellect, he was a psychotic madman who got off a lucky shot... and then was thwarted at almost every turn because he didn't think ahead. You say "imagine the possibilities", well I say imagine if we got the same Khan from Space Seed. Upon learning of Genesis, he was able to manipulate his way onto the Reliant, rather than capture its Captain with brain bugs. If he actually had a plan with what to do with it, and build an empire for himself. Hell, imagine if they didn't piss away Seti Alpha V, as the whole point of the episode "Space Seed" was to see (as Spock would say) what crop had sprung from the seed you planted today. If we actually saw Khan taming a world, as Kirk suggested. The whole need for revenge was pointless because Kirk gave Khan everything he ever wanted. Quoting Khan himself here - "A world to win, an empire to build"
moving on to Search for Spock... how exactly did it napalm anything? If anything, it further built upon the Vulcans in the same way "Amok Time" (TOS) did. And you know the two Leonards (L. Nimoy and Mark L.) had an absolute blast being able to delve deeper into the Vulcan mythology and culture.
On top of that, you have a story about the bonds of friendship, and a film with an ensemble who all get to participate in the story, rather than just sit at a console saying "course laid in Captain" or "hailing frequencies open" or "I'm givin' her all she's got"
Search for Spock doesn't get nearly the credit it deserves. This stood out even more for me when I showed someone Wrath, Search, and Voyage for the first time. Wrath was almost boring because of how much it dragged for the first half, while she got far more invested in Search and Voyage because she was able to invest in the characters. Particularly McCoy and Scotty (talk about someone totally wasted in Wrath, even with the deleted scene subplot about his cadet nephew)
A point I've made before - This pedestal we put Wrath of Khan on is one of the primary reasons why Hollywood continuously gives us "bad guys" and revenge in every single Trek film from Undiscovered Country onwards. If only we put Voyage Home on that pedestal, maybe we'd get more fun adventure than pew-pew