Personally, I think some of these don’t have much to do with listening comprehension. What @PeterBormann and @haskejl were talking about was academic specialisation. If I wanted to understand what was spoken by the vast majority of native speakers, I wouldn’t focus on academic specialisation (again, depends on your goal). As interesting as taking university courses can be and as useful summarising its material can be for writing practice, it seems to aim for a different goal than merely increasing your listening comprehension, assuming you are already at a high level. If you can understand the content easily already, this isn’t doing too much to increase your listening comprehension. Academic courses are usually single-speaker, clear, use reasonably formal language. In my opinion, this will do little for improving your listening comprehension. This is just like you saying you listened to a hundred audiobooks and saw very little improvement in your listening comprehension. You have to find challenging, difficult content. You have to go out of your way to find it.
You mention your focus is more U.S. related. The question is, Do you mean you want to focus on U.S. accents or you want to focus on being able to have similar listening comprehension skills as someone from the U.S.? Because Americans still understand English accents from Ireland, Australia, and India too. If you easily understand common U.S. accents, move onto other accents. Becoming familiar with other accents will improve your listening comprehension of U.S. content too.
I respectfully disagree. Native speakers understand a large range of accents, because they sat back for years and such content just fell in their laps. I agree with you that challenging audio material may not be easy to find in our content bubbles, but in large languages like English, it’s there. Local radio shows are on the Internet these days. Also, certain news programmes interview local farmers, bricklayers, etc., who likely have other accents (as accents in English are region and class based). You may have to go out of your way to find such content. Or at least VPN in to the Australian, New Zealand, Irish media channels, etc., if they have an geolocation block.
I don’t think there’s much to be overwhelmed about. The most important part is that the listening content is challenging. You may have to spend a bit of time finding challenging material, but it’s out there. Just like choosing reading material on LingQ with >5% New Words, “seek out challenging audio material” is a maxim, which will push your abilities no matter what level you are at. In my view, the method is simple. Just listen, ideally with background noise on increased audio speed. For a programme, you just need to find ~250 hours worth of challenging material for the year, ideally with multiple-speakers and a variety of accents.
P.S. Do NOT use subtitles or a transcript while listening. They become crutches on which one leans their weak listening comprehension on.