Personally, I would say 3-4 is a good gap. That is what we have done with our last 3 pups, I wouldn't want them to be much farther apart as I feel like they wouldn't play or bond quite as well when the older one is more set in their ways and has less energy for puppy shenanigans
Yeah, I got a puppy when my older dog was five. But the puppy died of cancer right when she turned two. So I just got another puppy right before my older dog turned seven. She definitely had more patience for the last puppy, but they are starting to bond and play a bunch. It's nice to see. The older dog seemed really off and saddened by the loss of the last puppy. Nice for her to have a friend again.
Depends on the breed / size of the dog. Large dogs have shorter lives. I like large dogs. About five years is a good age to get a puppy in that context. I wouldn't wait more than seven years.