While searching for something new to read on KU, I noticed that back issues of
The Magazine of Fantasy & Science Fiction are available with a
Kindle Unlimited subscription. You have to search for "Magazine of Fantasy & Science Fiction" (and possibly a year) because if you go to the F&SF Kindle magazine subscription page, there's no indication that back issues are included with KU.
Here's an example:
The Magazine of Fantasy & Science Fiction September/October 2018 (The Magazine of Fantasy & Science Fiction Book 135)
I tried some other SF&F and mystery magazines, but I couldn't find any others. It's like they're trying to hide them.
Anyway, if you signed up for the 3 months of KU for $0.99 deal and you're having trouble finding anything other than dreadful self-published stuff, this might be an option.