If not, are there other ways to effectively spend this time towards incremental self-improvement?

Reading textbooks in little pieces might work anyway; it does for me.

I can read textbooks nearly as well during a 20- or 30-minute train ride as I can in more stable situations. (Obviously I read less during a half-hour ride than in an hour spent reading at home, but my rate & retention seem similar in both situations.) There are textbooks I can't just read straight through on a bus, like maths & physics textbooks, but I can't just read straight through them anywhere else, either, because I need a pen & paper for the exercises. But a textbook I can read at home, like an oncology or sociology textbook, is usually a textbook I can read almost anywhere else.

If you've tried this already and found it not to work, ignore this comment! But if you haven't tried it, it's worth a go.