You sound very thoughtful. I don't know if you'll be living at home until you are 30 yrs old, you may need or want to move for work and finding a place that will allow a dog is always uncertain, so is your family OK with caring for a GSD while you are gone?
Getting a new puppy could make your Cavalier feel like younger again and he may want to play, or it could go the other way where he is annoyed with a puppy.
If you read through here, you'll see there is not just one type of German Shepherd. There are Showlines and Working lines. The Showlines generally have a milder temperament, the working lines may be a bit of a challenge for the first 2 years.
GSDs are very loyal and would love to have someone home all day. You may want to ask your vet if he/she thinks the activity of having a new dog would be OK. I'm just curious as why you want a GSD? We have a very active Greyhound rescue in our area, they seem so sweet and nice, I almost took one home, but we are at our pet limit. So just curious why a GSD and not one of the other nice breeds you've fostered?