Hi there! I'm glad you're enjoying the forum it's a small but awesome community!
First I just want to point out before someone on a certain UK Facebook puts it across rudely, Service Dog is an American Term. Service Dogs here are Police Dogs, etc. For disability we use the term (and the legal term is) Assistance Dog! :)
I had an Assistance Dog for Epilepsy, EUPD and Autism for a few years before he washed and I, and many members of the psychiatric AD community would whole heartedly not recommend a German Shepherd. German Shepherds very commonly feed off the handlers emotions and depending on the lines they may feed off a meltdown and/or a panic attack for example and become reactive to members of the public trying to help and/or paramedics. ADUK have cut the number of GSD's because of the extremely high wash out rate of them.
Here the recommended breeds (called the fab 4) are Labradors, Golden Retrievers, Collies (Rough or Smooth, Smooths are the choice here but roughs are the choice in America) and Standard Poodles. I will say when it comes to poodles you'll have to choose the lines carefully because some can be sensitive and not a good fit for Assistance Dog work, they're not going to bite like a GSD but I've seen a St. poodle Assistance Dog that started whining and peed itself out of anxiety when the Handler had a panic attack and there's been a few St. Poodle wash outs due to feeding off emotions.
In general reputable breeders won't allow you to assess the puppies. The breeder of mine allowed an independent behaviourist to come in and help with the decision, but in general breeders are around their pups and the parents 24/7 and will know the temperaments and potential a lot better than you or an outside trainer/behaviourist. Most reputable breeders also breed for work or show and so the assessment will have to take place between 8-12 weeks because that's when those breeders make their own choices based on conformation, temperament and/or working ability.
I really hope all this helps your breed decisions a bit and I wish you the best of luck with getting and training your Assistance Dog prospect!! <3