I too am looking to train a service/ assistant dog for. Am adopting a youngish heeler.
Have been searching for a service that helps with training but am also happy to do most myself.
With past puppies, dogs I’ve found asking/saying toilet as soon you get up, after meals, and throughout the day when you see signs they need to. Soon they’ll let you know, and sometimes wake you up pawing you or vocal sounds. (Not all are quick to catch on, I think I was lucky to have smart dogs). Crate training can help also as they won’t go were they sleep.
As with furniture, I haven’t had any that did that so not so much experienced.