The FIRST thing I would be concerned about is getting an Australian Shepherd (mini or not) when living in an apartment and having to leave the dog alone for several hours each day.
Are you experienced with this particular breed? If not, please reconsider your choice of dog breed. This is a recipe for disaster and neither you nor your dog will be happy.
Australian Shepherds are one of the most extreme working breeds out there. They were bred over hundreds of generations to run after stray sheep in the vast Australian landscape and read human signals and follow human commands for a whole day. If they don't get a comparable amount of training, stimulation and exercise from their owners - especially if left alone for several hours a day - they quickly find their own entertainment by chewing furniture, scratching walls and carpets, running around and barking nonstop and generally destroying your apartment. When you come home exhausted from work and just want to waste an hour in front of the TV, he'll constantly want to play and interact with you because he was bored the whole day.
Golden Retrievers are more balanced and well known for their agreeable personality, but they, too, were bred to accompany humans on hunting trips.
To get some ideas about games and ways to mentally and physically stimulate your dog, please read this question.
Any dog can develop separation anxiety. The biggest risk is unpredictability and a lack of separation. If you're around your dog all day because you work in your home office during Covid lockdown and then suddenly you disappear for 8 hours (because you have to return to the office), your dog may not know why you left, when and if you return and how to deal with the situation.
To avoid this, you should train being alone with him in incrementing periods of time. You can start with locking your puppy into a crate or a different room for 5 minutes. No matter what the puppy does (probably whining and scratching at the door), you only open the door after 5 minutes. You should either act as if nothing ever happened, or cuddle with him in a very calm way. You shouldn't encourage him to be overly excited to see you by playing with him or praising him in an overly excited voice.
Repeat this short training several days. Once your puppy learns to deal with being alone in a calm manner, you can gradually increase the time he's left alone to 10 minutes, 15 minutes and so on.
Don't forget to train leaving your apartment without him. If the only time you put on a jacket and leave the apartment is when you go on a walk with your dog, he'll be very excited every time you put on a jacket and become extremely frustrated if you leave without him. That can trigger separation anxiety or destructive behavior. To avoid that, simply go out for a short time without him, for example to go grocery shopping.
A great way to mentally prepare your dog for being left alone is to establish a word or sign that indicates he's going to be alone. Don't use a command like "stay", because you cannot release your dog from the command if you're not around. Choose something different like "goodbye" or "stay in here" and repeat the same words every time before you leave your dog.
You should not talk to your dog at length before leaving him. Some people do tell their dogs that they'll be gone for a while and their dogs seem to understand them, but in reality dogs cannot understand human speech to that degree. Some dogs even get anxious by this kind of behavior because they expect you to want something from them (maybe telling them about an intruder or a source of food or tell them to follow you). If you then turn around and leave the apartment, they try following you or calling you back because you just talked to them, you just indicated that you want something and if you leave now they won't know what you want.
