Interesting first impressions! I suspect you're doing your thing of too much, too quick, Josh.
It probably took me over half a dozen listens of the 4th over a couple of years to get to grips with it. I'm not suggesting your views aren't valid, or that you need 2 years, but I suspect your views might change with increased familiarity with the music.
Maybe Boreyko's 4th just hit it lucky for me, and I'd have had my "ah, so that's what it's about!" moment with Haitink or Gergiev if they had happened to be the one that was played when my brain finally managed to get itself together with the music. I'll try Haitink again. Not sure Gergiev has had any play time with me, so maybe I'll give that a go. Boreyko's sound quality is certainly a big plus for me. A coarse or over enthusiastic treble can get very wearing in music like the 4th!
Perhaps the difference between the magazine and Amazon buyer reviews is partly that reviewers are under time pressure to get on to the next review, and more immediate responses are the ones that make it into press, while Amazon listeners have more time to form impressions? I can certainly relate to Boreyko being more relaxed in the less intense parts, but in an hour long work of huge intensity in parts, the relaxation is a welcome feature for me. Or maybe not. I'll have some more listens to alternatives.
As for the 6th, I'm a bit shocked! I don't hear it the same way as you do at all. It's one of my favourite Shostakovich works! It's somehow different to the rest of the symphonies to my ear, I think there's a parallel with Sibelius 6th which also is similarly unique in his output and is my favourite Sibelius symphony. Neither of them blaze away at any point, have massive climaxes or are emotionally searing, but somehow they both get under my skin in their understated way and have a very deep affect on me. I think both 6ths took quite a while for me to appreciate.