My impulse to "mark mines" is so deep; yet what really matters is clicking on the non-mines and opening up the board as much as possible!

Go for the open space

In general this is a good rule. But in small spaces, and towards the end of a game, you should totally switch focus to thinking about how to open up the board as much as you can, rather than continuing the normally safe conceptual work of marking mines properly so that later chords will be efficient.

I find it really hard to keep doing this switch, and to let myself think about whether or not I should do it, without letting that be distracting.

Screenshot is from my minesweeper version here: https://minesweeper-friendly.fuseki.net/