I'm planning on taking this route. It's a little over 3 hours. I'm no longer young. Is there a toilet on board because it seems the bus is nonstop?
I have no experience with that bus, but it is typical for buses to stop every 2 hours or so, and this report from May 2017 indicates that the Tirano-Lugano bus does make such a stop.
It sounds as if you should avoid the toilet at the Tirano bus station, but the one at the train station will probably be OK (though conditions can vary).