I did dismiss The Deer Hunter but thinking back I'd say that that film had the makings of the types of scenes we would later see in ALIENS and Platoon. I think that's more of a "gateway" movie though, because so much of the actual film is really about before and after war life and not so much the war itself. Obviously ALIENS and Platoon are straight up war movies. But still, the action in The Deer Hunter was indeed pretty gritty and realistic. Not In Harm's Way though, that's not even close to what I'm talking about.
When I say "documentary" I guess I don't mean that literally or even that the film is trying to make you think you're watching a documentary, just the feel of a camera crew really being in the middle of a chaotic, disorienting firefight. I really don't think those kinds of sequences "arrived" until 1986. Kind of like CGI. We got glimpses of what it could do with the morphing sorceress in Willow and the Abyss water tentacle and T1000 but I don't think it really came into its own until Jurassic Park. So ALIENS was the Jurassic Park of war films and The Deer Hunter was The Abyss.
It sounds like you disagree but I don't think those older films you list qualify to the level of ALIENS and Platoon and of those two ALIENS came first so I do give props to Cameron. Even if he was in "Viet Nam mode" after Rambo he still used the momentum from Stallone's film to segue into his own movie with truly chaotic and terrifying war footage.