Thank you for the answer.
If you want some inspiration for the toggle: have you ever seen the Firefox uBlock Origin plugin UI? They basically have a giant on/off button, which I think is a rather good solution.
Interesting approach for the difficulty evaluation, and rather pragmatic. If you happen to have some write-up for the grammar heuristics or more detailed sources, I'd be interested :)
Definitely didn't realize I had to set difficulty manually (but maybe I'm just blind). Would be interesting to see if there's any smart way of auto-evaluating progress...maybe something like occasionally asking how difficult a sentence that the user hovers was or even just passive signals such as whether sentences are hovered. Just because if difficulty would be evaluated by the software, you'd both have a non-subjective measurement of your progress and you truly just could have this on in the background, never thinking about it, and still always learn.
Just some random thoughts; in any case good luck with this!