When you review while walking are you just looking at your phone? I’m wondering if there’s some less obtrusive way it could be done, like via smartwatch or even 100% by audio or something.
I currently use a bespoke piece of software for this now but I've had great success in the past with Flashcards Deluxe (by Orange or Apple). It supports automatic TTS, answering by gestures, etc. I used to use it back when I had a long commute.