The first film in the trilogy is quite good, setting up many battles, intrigue, and character depth. The second film falters a bit, due to building the story rather than continuing in non-stop action, as a second-of-trilogy ought (in my book). It's cool that the nano-metals are used for the Vultures, but we never see Mecha-G in form. The third film disappoints, for me, only in that the monsters are limited in range. Mothra is seen and not heard, Ghidrah is missing his lightning bolts, and Godzilla is exceeding slow (yes, he's 300 meters high, but that's a reason, not an excuse).
OK, so let's talk about why this movie was good. Many have commented that Haruo ended the film hate-filled and as a poor messiah, more depressed than righteously angry, and left his child behind for no reason. I disagree. There were so many references to the metaphysical Messiah that it should be impossible to miss that Haruo simply believed his own hype. He was guided by Metphies, from a young age, to believe he was something special. This is why Metphies gave him extra rations, and made Haruo's plans seem like the best course of action (even if they were). Notice that it didn't faze Haruo much that he was to aid in being the conduit for Ghidrah, even as it dawned on him that this meant the end of Earth.
Haruo was actually an anti-hero for most of the movie. He enjoyed his role as commander a bit too much, seemed reckless much of the time, called a hero a coward, had two women at one time (against the hero trope), and was generally loud and obnoxious. He kind of liked the struggle. He liked the conflict. He didn't even buy into his peaceful parental memories, or his future as a father. Instead, he wanted to continue the fight, go out in a blaze of glory, be known for what he did (there is, of course, no reason to believe he destroyed Godzilla at the end, anymore than we should believe Ghidrah is really gone forever).
Contrast with Godzilla, who is the manifestation of all striving towards prosperity, the target of the Buddha, if you will. Ghidrah is portrayed (I would say wrongly) as the natural enemy of that striving. In fact, Godzilla is portrayed as the PROTECTOR of Earth, because it was Ghidrah who destroyed Earth, not Godzilla. Godzilla merely re-adapted Earth for life after the destruction, simple, even prehistoric, as that life may have been. Haruo did not want that simplicity and peace, apparently. Haruo was actually the enemy of Earth, the conduit for Ghidrah, the enemy of the protector Godzilla.
Godzilla was not on a rampage when they arrived back to Earth. In fact, and this is important, Godzilla attacked not the Earthmen, but the Exifs on board, because he KNEW, instinctively, they were bringing Ghidrah back. If Godzilla were a bit more proactive, he would've stalked the Exifs before any ceremonies and sacrifices could take place, but that would've stopped the film's tension and build-up in that area. I will say this also: DESTROY ALL MONSTERS is still better, no matter how silly you think it looks next to this anime. I was not really impressed that the humans in the trilogy were so easily led down the path of cult religion. But I assume this is part of the social science which presently corrupts much of modern cinema. In the 1960's, even the 1990's, films, we do not endure mankind as sheep to the slaughter, but rather faked out by gifts of alien science and promises of peace, plausible motivations. So in that respect, the anime fails, because Ghidrah could not come without their willing participation, their desire to be rid of Godzilla so badly they would take on an alien "god." This is my peeve with many movies: I don't want to be taught an anti-human message. "1984" is not anti-human, "War of the Worlds" is not anti-human, and "Godzilla" should not be anti-human. It can be anti-war, anti-pollution, anti-greed, anti-corporate, but not anti-human. So that part was a fail.
Now, if you watch to the very end, after the credits, there will be, must be, more anime, involving Mothra. However, I don't know that Ghidrah was charismatic enough for me to anticipate another fight between Godzilla and a more on-screen Mothra. It would be very difficult to beat the 1990's films, especially GODZILLA VS. KING GHIDORAH, if not REBIRTH OF MOTHRA III. In fact, I don't know the purpose of introducing a cameo Mothra in this film if there is NOT going to be another anime.
As to re-watchability, the anime misses several key components, which exist in most Godzilla films from the 1950's through the 1990's: (1) battles of immense proportion which repeatedly stun, (2) characters you love, or love to hate, (3) culture charm. I was not awed by the combat scenes, I did not love or hate anyone enough, and I missed the Japanese micro-moments.
So I give the series a 7 for not making Godzilla into just another Tyrannosaur, and for trying to give us a story arc which included nostalgic elements of the past, as well as a provocative, if typical, anime plot. In contrast, the new GODZILLA VS. KONG (2021) gets a 4 from me, due to that OBNOXIOUS child and her "mother", the paper-thin "villain", the RIDICULOUS cave with Kong's paw-print, and so forth. But the second-half battles weren't too bad, but then I immediately watched the 1962 version for the umpteenth time and smiled all over again. What can I say? .. Also, I wasn't very happy with the previous "new" Godzilla. I gave KING OF THE MONSTERS (2019) a 5, due to the DUMB woman who hooks up with completely-OBVIOUS environmental villain, and she is supposed to be a genius, and we need to overlook because she had "good intentions" and OH WELL, the world must be destroyed because of her. I did like the effects in that one, however.