Katherine, it's good to hear that even someone as highly competent as you needs to rework things now and then!
I also have ongoing large projects that don't need a lot of planning, and the small administrative stuff tends to pile up on my next action list. My solution these days is to schedule a morning a week to burn through it (usually it's At Desk and Phone Calls).
I've also been experimenting with scheduling a "big rock" for each day (adding some Covey method to my GTD). This sometimes works in that it helps me to choose a focus in my very flexible, multiple-focus schedule. I'll report in later when I decide whether to keep that piece.
Your one-action-per-card method reminds me of something I have my colored pencil art students do to get familiar with their pigments (there are many, many colored pencil colors!). You make a little swatch for each color with the name on the back so you can't see it, and organize them in groups by how you see the similarities and differences. You end up with a nice color reference tool (the Color Fan invented by Barbara Newton) but the process is invaluable for becoming aware of your perceptions: should this greeny-brown go with greens or browns? What else looks like it? Oh look, I have a whole bunch of blue-gray-violets. The categories arise out of moving the little pieces of paper around. Makes sense that it would be the same with contexts.
Good luck with your rethinking!
For reasons unrelated to GTD I am interested in this color fan. I googled it and your post came up number 1, with nothing else that seemed relevant.
Can you point me in a direction where I can find out more about this?