So for the most part, that’s what we’re here for and I can take a more hands on look at the project myself if it comes to it. There’s no “Hotline” but we do have other resources! For developers in the Unreal Developer Network you have a bit more access to developers of the engine themselves and a community of professional developers, though that’s for custom licensees. Another place to look would be the Unreal Slacker’s discord (unaffiliated with Epic Games). It’s a 3rd party discord group that has routinely answered deeper questions than I could, since it’s not just one specialized developer but a huge group!
Your problem is definitely one of a kind! You ran through most of the conventional wisdom about these issues on your own for the most part. So let’s look towards more environmental issues.
What are your specs?
Are you currently on DX12? If so, could you drop to DX11 and see if there’s any changes? Vulkan as well?
Are you material preview windows also doing this? (this can help determine if it’s only level/project deep and some other factors).