Thank you for posting this! This was very interesting to read.
I was originally going to post my own mini-essay as a counterpoint to yours, but going through the other comments, most of what I wanted to point out has already been covered. I'll try to keep this short and only talk about my two biggest issues with the film.
For the first point, I just want to talk about what the premise was and why it doesn't make a whole lot of sense. The premise boils down to Hiccup and the Berkians seeking to find a new home due to various issues that they're facing.
One of these issues is overpopulation due to all the dragons that Hiccup and the Dragon Riders have brought back to Berk. What I don't understand is why overpopulation was ever an issue to begin with. Hiccup and Fishlegs have spent literal years learning about the dragons, including their natural habitats, migration patterns, etc. as seen in the TV series. We've already seen the Dragon Riders transport dragons (rescued or otherwise) to various islands that would make good habitats for them. Think of Garf the Deathsong, the Snow Wraiths, and more. In fact, overpopulation and relocating dragons was the main thing we saw in Defenders of Berk Episode 8 "Appetite for Destruction". During that episode, when the Screaming Death starts sinking entire islands, Hiccup points out that he's worried about overpopulation becoming a problem for Dragon Island. For some reason, this is completely ignored in THW, and Hiccup either doesn't notice or care. The point is, overpopulation being an issue is completely contrived considering Hiccup is careful when it comes to these things.
Second, we have the issue that apparently the Dragon Riders are becoming "too reliant" on their dragons, at least, according to Valka (which is ironic considering she's been dependent on Cloudjumper for at least 20 years). This is pointed out by Valka after she witnesses the Dragon Riders' sloppy performance during the raid at the begin of the film. But here's the problem: The Dragon Riders are way more coordinated than what we saw in the film. If you take into account all the times the Dragon Riders have raided Dragon Hunter ships during Dragons: Race To The Edge, or even just launched any kind of attack on a Dragon Hunter base, there's no way to reconcile that with what we saw at the beginning of THW. Also, a good example of the Dragon Riders working as a coordinated unit is RTTE Season 4 Episode 4 "Not Lout" from the 18:42 mark to the 21:26 mark (I'd recommend a rewatch of that scene, followed by a rewatch of the opening scene of THW; can you really say those are the same characters?). In the THW raid scene, the Dragon Riders can barely do anything without their dragons' assistance. The point here is, the only reason the Dragon Riders' raid was portrayed as being so sloppy was so that Valka could later tell them that they're becoming too reliant on their dragons. This is a textbook example of a contrived situation.
Third, the threat of Drago's Warlords and the remnants of his Northern Alliance. They send Grimmel after Hiccup and Toothless, hoping to get rid of him. This lone Dragon Hunter/Trapper and his small group of dragons invade Hiccup's house and set some buildings on fire. Apparently that is the tipping point that pushes Hiccup to leave Berk in hopes of finding the Hidden World. Apparently, this one incident makes Hiccup and the other feel that it's no longer safe to live on Berk, just because their enemies are not attacking them head on. This is far from the first time that Berk's enemies have attacked them at their home. The Outcasts, The Berserkers, and most importantly Drago Bludvist himself, have launched far worse attacks on Berk that the Berkians fought through and prevailed.
Hiccup is portrayed in RTTE is a brilliant strategist and tactical genius by the time the show ends. The way he acts in THW makes him look like a moron. Consider this: Hiccup got his entire village to abandon their home to go looking for a place that might not even exist. Remember, Hiccup mentioned to Astrid that the Hidden World was a "mariner's myth" Stoick had told him. It was a literal myth. There was no evidence that it actually existed. At the very least, Hiccup should have taken the Dragon Riders to make sure the Hidden World was a real place before getting his entire village to abandon their home. And even then, there was no guarantee that the Hidden World would be a livable place for humans. Another issue that could have been resolved if they'd just gone to check and see if it was a real place to begin with. Hiccup's OOC nature kind of ruins the film. This also leads into point number 2.
Finally, going into the second point, THW goes against everything the franchise stood for. Hiccup's story has been one about persevering against impossible odds and doing the right thing, even when the world tells you that you're wrong. Hiccup has always been an idealist. And he fights for his ideals (believing that the world would be a better place if dragons and humans co-existed) with everything he has. THW tries to be "realistic" by conveying a message of learning to let things go. While learning to let go of things is a great message and an important life lesson for people to learn, it has no place in a franchise like How To Train Your Dragon. "Realism" has no place in a franchise that has always been about creating an ideal for others to strive towards and fighting for those ideals. In many ways, the complaints about Hiccup in this film are very similar to that of Luke Skywalker from The Last Jedi. That being, portraying a character as idealistic vs. realistic. Hiccup has faced off against many adversaries, many of which are extremely threatening, such as Viggo Grimborn, Trader Johann, and Drago Bludvist. Hiccup literally lost his father while facing Drago, which would have been enough to break anybody, yet Hiccup's conviction remained unshaken and stronger than ever. Because of this, it's hard to believe that a single Dragon Hunter like Grimmel would end up being the cause for Hiccup giving up his fight to create a better world for humans and dragons alike. It feels like THW tried to force the ending from the books onto the franchise, which doesn't make much sense at all, considering how optimistic the franchise has been, compared to the dark nature of the books.
Don't get me wrong, I really enjoyed reading your essay, but I can't say that I agree with a lot of your points. This was longer than I planned on making it, and I apologize for that. I hope this didn't come across as rude or anything. I'm interested to see what your reply is.