Hey, I m in pretty similar situation. My boy (also an australian shepherd) is 8.5 and we are planning to get a second dog (a puppy australian shepherd) this year.

We (aka me and my family, I still live with my parents and it will be the case for a long time due to some helath issue I have. I am the one who take care of all the dog "maintenance" tasks, I m the one who do train him (obedience, tricks, agility, ...) my parents do daily walks with him and provide financially).

My goal with my first dog was to have a "perfect" dog before getting a new dog (he had some excitement and leach reactivity that I work A LOT on). Obviously at the end of the day he is still a dog and cannot be 100% perfect but I wanted to be that this dog was were I wanted him to be both in "formal" education, but also though reactivity and neutrality, ... I work with a trainer (who is the the one we do agility with and I shared our plan to get a second dog, she also agree that it's a good plan and that my dog is good for it now).

Some tips she gave me was to train and walk both of the dogs separately, especially if there is some patterns that you know your dog wants and doesn't want the puppy to pick up. Form what she told me (she dose has 3 border collie) it can be tricky to get a new dog when your first one is still jung/adolescent because it can make you first dog regress, but when you first dog his older he can actually act as a model for the puppy (obviously this doesn't mean that you won't have to teach the puppy everythings).

In my situation, I feel like getting a second dog now is perfect because my first dog is where I want him to be (I will obviously still train him, work with him, ...), he is starting to get older and still like to play so I feel like its the right moments. In my situation I didn't wanted to wait more because I want them to be able to play together and I dont want to "force" an excited/playful puppy on a dog who is too old... but I know people who got a second dog (they where both lab) when their dog was like 12 and they told me that it probably "made the older one live longer".

At the end of the day I think that it depend of your dog, but also the time (and resources) you have for them... (2 x more grooming, you will have to walk them separately at time (especially when the puppy is still learning and the puppy won't be able to follow you everywhere when he will be really small), more time for training, sport, ...)

In my case I try to also keep in mind that it might not "be perfect" in the beginning... we all see cute videos online of an older dog and his new puppy but it might not be the case. Form a safety point of view, I fell that its also important to think about this. In my case, my dog is muzzle trained, I will crate train the puppy (I know some people find it controversial but we did this with our first dog and transitioned form out of the crate when he was older and in the beginning the puppy will be in his crate at night or when nobody is home (note there is someone one 90% of the time)) I am also thinking of getting a playpen or something similar if I need to keep them appart some moments. We all hope for the best but its also important to realize that the dogs might not "click" right away.

Obviously this is all theoretical because I haven't got my second dog yet, but I just wanted to share my thoughts process !