Thanks! This post reminded me to check the physical stuff, not just the many layers of configuration. Sure enough, the X stepper motor itself was lose. Prints are much better now! (In my case, the skewing only occurred late in prints. There must be more resistance on the nozzle once the model is higher.)
These were extremely helpful and improved my semi flex prints a lot. (I was printing too fast, with too little material.) I'm still using a .1 mm layer height and .8 shell thickness. Do these seem reasonable in your experience?