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I write short-fiction reviews for Rocket Stack Rank, so I regularly read and review all the stories in the 11 major SF/F magazines, where "major" means those regularly covered by the main short-story reviewers and those whose stories frequently turn up in big anthologies and/or are short-listed for awards.
None of them includes any resources for writers that I'm aware of, but most of them have regular interviews with writers and artists.
(Links below are for how to get Kindle subscriptions, where possible. Note that many of these are actually available free online, but by subscribing you enable them to stay in business. It's a lot more pleasant to read on a Kindle than on a PC screen anyway.)
If you want high quality stories and writing, you want either Asimov's or F&SF. Both have a mix of fantasy and SF stories. Neither one shies away from darker stories, but that's not their focus.
One excellent magazine that's often overlooked is Interzone, the main UK SF/F magazine. It has excellent stories, high standards, and tends to be a good bit darker than Asimov's or F&SF.
None of the other eight major SF/F magazines seems to quite match what you're seeking, but I'll say a bit about each one just in case.
<em>Analog</em>. Has a hard-SF focus, but sacrifices writing quality for the sake of cool scientific/technical ideas. It has plenty of post-apocalypse and dystopic stories, although it's pretty optimistic on balance.
<em>Apex Magazine</em> certainly meets your requirement for darkness. About half their pieces are horror of one form or another, and most of the rest is dark fantasy or dystopic SF. Not all, though.
<em>Beneath Ceaseless Skies</em> is fantasy-only, much of it secondary-world fantasy. The writing and stories are quite good, and it can be pretty dark (they even run some fantasy horror).
<em>Clarkesworld</em> is pure SF, but it goes for sense of wonder more than anything, and it too will overlook writing problems if the story has enough "wow" factor.
<em>Lightspeed</em> offers an even mix of SF and Fantasy, and the diversity of story types is so great that no one has been able to pigeon-hole it. The editor is one of the best in the business, so the writing quality is quite high.
<em>Strange Horizons</em> has plenty of dark stories, although their focus isn't so much on dark stories as, well, strange stories. They're the only magazine here that's a charity, so you cannot subscribe to it but you can make a tax-deductible donation.
<em>Tor.com</em> gives away free stories to give readers a taste of the work of authors of novellas, novels or series sold by Tor Publishing. The writing quality is professional, since it's edited by Tor's professional editors. These stories tend to be more optimistic, although they do run quite a few dark stories. Tor.com is free online and you cannot subscribe or donate even if you wanted to.
<em>Uncanny</em> tends towards darker fantasy with a social message. Writing quality is generally pretty good, but if they like a story's message enough, they'll overlook bad writing.
There are literally hundreds of other venues for SF/F beyond these, of course, but these are the core publications.