I would have a trainer temperament test your dog. Danes are not a common choice for several reasons, and even though you want this dog to be your SD, they may not be well suited for the job. You will sink money into the training, and they have a higher probability of washing.

I would also wait to get your dogs hips checked and get a clearance by the vet before diving in to train this particular dog. A SD's lifestyle is hard on the joints, and for a Dane to be lowering themselves to crawl under tables and lay on hard surfaces is often is not ideal for their health.

Danes will always get people's attention, even when not training as a SD. Evaluate if this is something that will worsen your symptoms, because if so another breed might be a better choice for you.

I am a SD trainer and I keep Danes as pets, but in a majority of circumstances I do not recommend them for SD work, especially if you don't NEED the extra height for tasking.

Also, no part of owning a Dane, or of SD training is cheap, so make sure you have your parents support on this journey, physically and financially- because otherwise, as a 17 year old, the cards are seriously stacked against you.