I understand that you want to go in fully prepared to ask the right questions.
I am glad to hear that Toby has maintained his muscle mass, he must be a very active fellow. We often see loss of muscle as dogs age and as such that can affect their ability to get around.
I don't think that his hearing loss is related to either medication, or a low potassium level, that is not a noted side effect. Hearing loss related to old age nerve degeneration is actually a very common old age change that I saw with my own 15 year old shepherd. They are so smart at reading facial expressions, and body language, that we often don't realize how much hearing is lost until they are almost totally deaf because they are so great at compensating just by paying better attention to us.
If his potassium levels are up then we can use fluids to bring them down and lower his dose of medication to help keep them down. This is quite rare, and I've actually never seen it with Enacard use, but it is a noted side effect so I mentioned it to be complete. More commonly we see mild hypotension and that is also very treatable by lowering the dose.
I am glad to see that you are using a harness, that can make a huge difference in tracheal irritation versus a collar. I also recommend using a humidifier, especially in the room he sleeps in. Humid air is very soothing to airways.
I'm sure that they have told you oral hygiene is extremely important for him. Any gum or tooth infections, plaque or tartar, lead to inhaled bacteria which worsens airway inflammation and tracheal collapse and bacteria in the blood stream which can lodge on heart valves or in the heart muscle. Daily oral anti-bacterial rinses and brushing if he will let you go a long way to improving his health.
If he is a fellow that gets lots of ear hair, waxy debris and ear infections then plucking his ears to allow better air circulation is probably best. If he's never or only rarely had an ear infection I wouldn't make a big deal of it, pull what is easy to remove but it doesn't have to be completely clear. Removing hair and wax improves the ability of sound waves to travel in the ear but if he gets very upset when you pluck you need to balance the coughing and the damage that can do to his trachea with the benefits of sound waves getting to an ear that is likely mostly affected by nerve degeneration (not much).
Best of luck with Toby, he is a very well loved and lucky boy to have you looking out for him.