Worry about training for the very specific tasks, later.
This bit.
A good vet clinic will run puppy pre-schools, which will show you how to get pup to focus, sit, stay, drop and come. From 6 months, pups are fully immunised and usually welcome at your nearest obedience club. Those clubs are usually very cheap to attend (I think our local has a $20 annual membership and a $2 fee per class, which is about standard), and that will get you an obedient, easy to train pup ready for harder tasks and PAT testing preparation with very little outlay in training costs.
It's at that point you'd engage with your preferred AD organisation. If you've selected a working breed, they'll train pretty easily (labs - there's a reason they use them everywhere, for everything!), and if you choose a breed bred for good temperament (again, labs - prefer a lab to a golden retriever in your situation since Goldies get much bigger than gold and black labs, brown labs can get very big) and keep it socialised, you'll likely have no serious issues come up.
Your preferred AD organisation may be able to suggest a good breeder. Certainly ours has a few breeders we commonly recommend, one of which is producing some really brilliant labradoodles (which can be a disaster if you get one from a breeder that doesn't know what they're doing), which are slightly smaller than a golden lab, but super friendly and wired to love working hard for their handler.
A puppy in the house is a complete game changer. You noticed that even with the short time you had Boz, so maybe don't get too hung up on teaching the special skill sets at this point. The single biggest issue is to help your mood, and put a smile on your face when you wake up every morning. Which doesn't take any obedience training at all! So anything the dog can be trained for on top of that? Is bonus.