My spoiler free thoughts correlated, spoiler tagged still for people that haven't seen it yet and want to go in blind as I have raised some of the themes explored.
[SPOILER="Godzilla"]To start off with, I will outright call this movie art on the same level as Shin-godzilla for me. Hard to say which one I prefer but they are both in my top 3 along side Godzilla 1954. Absolutely no scene was wasted in telling the story and nothing was lacking in the execution. The acting was phenominal and I really felt bad for the main character living with the worst case of survivors guilt I have probably ever seen. And we do sympathise with him, we see what he goes through and how others treat him harshly, and his struggles to put his ghosts behind him.
I have heard it said that this film doesn't have any political undertones but I disagree, I think there is quite a bit of silent political commentary underlying this film as well. It's not often thought about in the west but the idea of being a Kamikaze pilot really is scary, you are literally being ordered to kill yourself. If it was me I would also say no without hesitation too, screw the emprah I'm out. Though this concept was prevelant in Japans history it kind of goes without being said, but at the same time shines a light on this all through out the movie.
Another point I think is prevelant is that for me, this also feels like a reimagining of Godzilla 1954. The themes are actually very similar in that it's about post war trauma, except while 1954 was about the atomic bomb, this instead explores ptsd living in post war Japan from a soldiers point of view.
Godzilla himself feels like an amalgamation of different versions we have seen before. To give a poor analogy if this was Dragonball Z then he is the Perfect Cell version of the Godzillas. He has a sort of mix Showa/Heisei design, Millenium's raw power, Shin Godzilla's invulnerabilty (without his evolution powers) and a dash of Legendary Godzilla's fun beam sequence. It's completely OP but the movie deals with it in an imaginative way.
If I had any problem with this film it's that it has retroactively ruined any hype I had for Godzilla x Kong since there is no way it can live up to this one. There is a little bit of a sequal baiting at the end so we will see what they have in store for this new iteration of Godzilla.[/SPOILER]