Mac: DRM removal for Apple TV-purchased movies?
This is going to be hard to accomplish using Apple SDK and tools for multiple reasons. Setting aside any reason why Apple made it technically hard, it is for reasons.
What is not technically hard is using HDMI capture hardware to make your grabs. Or better, find a like-minded community that is sharing the burden and cost of getting high quality still frames for educational and legitimate and legal uses. (Also, what is legal is often something that’s not moral or even good for society.) Crowd sourcing has many benefits here in getting around technical hurdles.

https://film-grab.com/category/1976/
https://www.walmart.com/c/kp/hdmi-capture-devices