I think the portrayal of the Soviets (which, yes, is more extreme than Tom Clancy) is a symptom, not a cause of the problem here. I’m only two episodes in though note.
The problem is that for whatever reason, S3 is written wildly more cartoonishly than S1 and somewhat more than S2. Normally we see the reverse of this in shows. Often the first season of a show is basically a bit crude and cartoonish as the writers find their feet, find the characters, get some nuance and complexity going, and so on.
Here we’ve see absolutely the opposite. All the characters seem to have experienced either a reset, or a simplification, and much of the dialogue has become sort of in-joke-y in a way that isn’t in-jokes with the kids, as much as in-jokes with the audience. A lot of the scenes are ridiculous and overwrought in a way that makes the show seem artificial where S1 had a considerable degree of natural-ness to it. You’re spot on re: the Byers family. They felt real, they felt at risk in many ways. There’s nothing quite like that here. Everything in S3 is over-the-top. If you’d shown it to me without the context of knowing about S1 and S2, I would have been absolutely certain this was the TV show of a Mark Millar comic book or something, and relatively true to the source material. Characters have actually lost nuance, and that’s both unusual and bad.
The writing feels different because it’s sort of chummy with the audience, expects you to know what’s going on, rather than being extremely mysterious and creepy which is what made the early seasons work. Two episode in and the sense of peril is non-existent, too. There’s also a lot of crude attempts at being shocking, which were not really present in earlier seasons. They seemed to be there to make up for the inability to create real tension or peril.
About the only believable/human character on the show so far has been the old lady who caught the rat in her basement. And the Russians are the worst, because you’ve said, they’re just Bond-villain grade stereotype characters. Oh a big Russian dude who strangles people whilst looking nuts? Oh people executed on the spot for failures, in 1984? Could we have any lamer cliches? This isn’t Stalin’s era, and even then “on the spot” was an overstatement in most case. You couldn’t away with just murdering people in public in the Soviet Union in 1984. If they’d sent him to a gulag or something it would be perhaps plausible, but no, we got the full Bond villain ridiculousness.
Another bit that was pretty ridiculous was the toys being moved by El, which was very '80s movie-ish, I admit, but she’s got a nosebleed from it and no-one notices or case and it seems like it’s just there to show who is doing it, not to be sinister? Jesus. Wtf show? Nosebleeds from psychic powers aren’t generally a good thing.
And yet another example of cartoonishness has been the use of music, which has been strange and clumsy in certain ways. Like with the Soviets they’re playing the Soviet anthem over the baddies walking away and panning over the facility, but it just seemed inappropriate and clumsy, like we didn’t know these were Russians (?!?!!? maybe some of the audience doesn’t recognise Russian accent, Russian text, or Russian uniforms - but are they going to recognise the Soviet anthem?). Then with the mayor being an idiot they’re playing what, was it My Country 'Tis of Thee? one of those American ones, I forget which, and they’re playing ridiculous tension music when the kids are sneaking up Dustin. I get the joke, but it’s an entirely meta one that doesn’t really work. That whole scene could have been done vastly more effectively.
It’s kind of unfortunate that S4 has been so successful, in my eyes, because it means they have no motivation to change/improve S4. Indeed, just rationally they might consider doubling down, on the broad, comic-book sensibilities of S3. I hope they don’t. I hope they go back to creepiness and mystery, and keep the Soviets out of it.
It’s particularly difficult to watch, I note, after watching Halt and Catch Fire (most of the way through S2, still really great), which has a largely realist approach to the 1980s, indulging in a bit of nostalgia (“Remember those sticky octopuses that people threw at things?” “Remember how fucking huge Terminator was with younger people?” etc.), but remaining grounded and being true to the actual issues actual people had in the 1980s, how they talked, and so on. Then you go over to Stranger Things, and it’s like a ridiculous lavish cartoon of the 1980s, which feels like it’s constructed of '80s tropes more than an actual love/understanding of the '80s.