> Once you know what all the people in a top publishing company do, the difference between an amateur publication and a professional one becomes immediately apparent.

Any advise for developing this sense?

I will never work in a top publishing company but I have been able to approximate good design by first studying the fundamentals, then reproducing the layouts I see in popular media. I can make text into a beautiful book, and I see poor design choices in the corporate communication billion dollar companies.

But it feels like there’s a lot more I don’t know, and you never know what you don’t know, and it makes me wish I could absorb more from working under an expert.

There's no substitute for apprenticeship (by whatever name). Unfortunately, skills of this kind may be close to extinction. For someone like you just interested in getting better at layout design, I'd recommend something like 'The Elements of Typographic Style', by Bringhurst; this concentrates mostly on books, but much applies to other layouts. Of more general interest -- i.e., beyond layout design -- might be 'An Encyclopedia of the Book', by Glaister. There's a wealth of valuable design and print resources from the '60s - '90s if you can find them -- some libraries still have high-quality examples, but most have replaced them with much less-valuable contemporary resources. Look for book and magazine sales by university departments, businesses, etc.

The trouble with our age is that, despite the abundance of intermediate-level information, expert teachers in specific, and shrinking, professions are as hard as ever to access, if not more so.