I'm in the same boat with the text files in a dropbox folder. I thought about just emailing myself notes to a notes-only account, but I'm not sure there much of an advantage there over Dropbox. Still, files in a folder checks all the boxes for me. Compatible with everything forever, can organize and reorganize as creatively as I want, lightweight, can search my notes and sort a bunch of different ways, and osx even supports tags.

Granted, this is a little clunkier on mobile, but for quick notes I use drafts which is the speediest note taking app I've found (opens directly to writing prompt, one button save or append to dropbox files).