For something like this, I wouldn't try to lock too many things down. The pragmatic question is: can you give students a tool that makes them genuinely engage with texts more? This, to me, is the key. That's the only thing I would try to measure, because everything else will distort the picture and give you a false sense of knowing anything.

If Easify (or any tool) is genuinely useful to students then they'll use it. If it isn't, they won't, not really. They'll go through the motions, and whatever you try to do to capture them, they will evade, either now, or soon.