as the parent of a teenager who also knows everything
i try to balance him having the independence to pick food and also giving him healthy meals and choices....he is also taking a high school cooking class and could help me prepare healthy dinners if we wanted to do so...he stays active with an after-school job at the movie theater where he is on his feet most of the time...and now that he has a paycheck, i am really working with him on the food choices that he spends his money on - for example, a mcdonalds meal on the weekend isn't great, but not horrible, where mcdonalds every single day IS unhealthy.....moderation.....things like that....he has ADHD and impulse control issues so it's a learning process to do things like make healthy choices or control a portion size or stay out of the kitchen in the evening....and it's a lifelong process, because alot of us as adults still struggle with those choices
for a teenager with OCD and anxiety, i think you have a good idea to put her in the drivers seat as much as possible....that way she can plan ahead, prepare and know what to expect...i don't have those diagnoses, but even I don't like being thrown for a loop when I'm expecting something else to happen instead
i don't know how much help that is...but I think you're on the right track with ideas