I don't think I'm alone in having completely missed the tell to grab enemies out of their armor (for both the boss and the shield guys in the previous levels)
You aren't the only one, so that one's a bit more forgivable, but people are quick to assume that if they can't do something, it's impossible.
and the fight being infinitely longer than any you've face before (if you've faced any at all), so people just try to rush him down
While this makes sense, this bugs me. We all actively acknowledge it's a boss fight off-rip, so why is the first instinct to not treat it like a boss...?
quickly learn that the 3rd combo hit is nearly unuseable in most situation, completely giving up on it, relying on 1st and 2nd hit which do pitiful damage, makes the fight take even longer, leading to frustration and impatience
Completely understandable, but something that should be mentioned: the third hit can land reliably quite often, in fact. You have to just be aware of certain cues and keep track of how long animations may take. (Also, reminder that it can be cancelled before you leave the ground, so that makes it a lot harder to get punished for.) This is not something new players would know, so this is definitely a noob trap like you said, but it also kinda acts like that bar for entry I mentioned a little while ago. The game essentially says "if you can't learn how to play the game well enough to beat this boss, you aren't going to survive past this point.)