Hey there, my husband also has Stargardt and he's been going through the same things as your dad, so I know a bit of the helplessness you're feeling! You sound very considerate and compassionate, your dad is lucky to have you!
My husband is an avid reader as well (and also dislikes audiobooks!) What has worked for him so far is getting a Kindle and using very large font. He prefers the Kindle without backlighting, as his eyes are pretty sensitive to light. He can also read things like letters by using a magnifying glass. If your dad doesn't have either of those I would definitely suggest getting them.
Try to get him to go to a retina specialist so he can be in the loop for any possible clinical trials, and
$1 clinical trials to see if there are any near you that he may want to be involved in. My husband still wears contact lenses to sharpen the vision he has left, so make sure your dad still sees a regular eye doctor to get his prescription checked.
That's about all I can think of, but DM me anytime if you have questions or want to vent. My husband is 35 and has been diagnosed since he was 21, but Stargardts is different for everyone so I don't know if he is further along in his vision loss than your father or not.