I personally love the sound of electrostatic speakers and would try an affordable pair of electrostatic headphones as a step up from that class of open-back dynamic headphones if I were in the market today, but I don't listen to headphones as much as I used to.
I don't hear a lot of difference between the Apple headphone adapter and jdslabs' "Objective DAC". The $10 Apple adapter might even be better than the ODAC, but it only works with my phone.
I haven't used Spotify in years; I think I never reinstalled it when my old laptop died in 2018. I could never get past the audio quality of their free tier, and never tried the paid tier. Maybe it's better now. Their recommendations back then for me were really odd; I can understand why some people would complain. I am about the only person that likes Amazon's streaming service but their recommendations are just bland-to-bad. I've noticed Pandora channels have become much more repetitive. Maybe the whole industry has been pushed by their metrics to some horrible least-common-denominator point.
There's an argument that discovery is an unsolved problem that doesn't pay. There was a discussion a while back on this here: https://news.ycombinator.com/item?id=29474762
Subjectively, depends on the person. A majority prefers HD600, while some swear by the HD650, a colored (less neutral, warmer) version of it; I do prefer HD600 myself, while I've heard both extensively.
But planar magnetic headphones tend to be quite heavy and bulky, and you might be willing to sacrifice planar magnetic sound quality for comfort.