In the case of playing a random piece of music, if you don't know it, again, how do you know how well you've done? I've been contemplating a similar thing with my repertoire playing, choosing a random bar for revision or learning via Anki.

Another thing - it sounded like you already played your instrument well, but wanted sight read, when you were doing this learning. So perhaps you already had a lot of the phrases in your fingers so to speak and this was just allowing you to connect the notation to what you already knew how to play. Do you think this would have worked well in my case where I could play a different instrument but didn't have any mechanical memory of chord shapes, arpeggios and so on in my fingers?