Determined that most sub-$1000 closed back headphones are only a nominal bump in quality over what I'm using, and I've concluded that going in-ear/earbuds isn't for me either. So after spending time today looking at options, wading through advice here and elsewhere, and pouring over reviews and various forum posts I've landed on the Audeze LCD-2 closed back. They're $900 new, so I'm hoping to find a lightly used pair for less.
https://www.audeze.com/products/lcd-2-closed-back
It's MUCH more than I wanted to spend, but there's a real chance these could be the last good headphones I'll ever need to buy and music's important enough to me that $600 or 700 to maximize enjoyment of it is justifiable. I also spend as much or more time listening to music on headphones as any other source, so it's the money being spent on the 'right' equipment.
I also plan to buy some extra ear pads to toss in the drawer to ensure that longevity (finding replacements could prove difficult as decades pass).
Huge thanks for the input, everyone's ideas sent me in differing directions that were all necessary for arriving where I did.