2 little things:
What's wrong with mechanical metronomes?
My Grandmother gave me one, and i personally actually like it.
The sound of it is less annoying than the sound of some electric ones,
and they are as simple as it can get, there is no way someone can do something wrong,
just slide the weight to the desired speed and let it tick away.
I think there are also some out there that make a special noise every 4th beat or so,
but i'm not sure.
And they look nice.
Almost forgot that: No batteries required.
It would have been nice if you maybe would have shown free alternatives,
like the Minituner, or one of the various online-metronomes.
Those are the 2 things that just came to mind,
but i really liked the new lessons till now, great work!