Here's a list of resources I've found the most useful over the years. Hope you find something new!
Ari Arnbjörnsson's Unreal Engine notes—eclectic yet extremely useful collection of odds and ends. Highlights include best practices and practical debugging tips.
Ben UI website—a treasure trove of Unreal Engine articles, documentation, and code snippets across various topics. UI is a big focus but there's a lot more than that in there.
Tom Looman's articles—these are all highly practical and of very high quality.
PrismaticaDev YouTube channel—want some fantastic, easily digestible tech art content? Look no further.
Optimizing TArray Usage for Performance by Zak Middleton—even if you don't need to squeeze out every tiny bit of performance out of your arrays (and in most cases you really do not!) this will help you understand what's going on under the hood a lot deeper.
Optimizing TWeakObjectPtr usage by George Prosser—if you're not using TWeakObjectPtr's you really should start! And this article will give you practical advice on how to do it best.
UE4 C++ Interfaces - Hints n Tips by Steve Streeting—most extensive writeup of interfaces I've seen. While I haven't tested some of the last few points in new engine versions, the basic facts seem solid still.