I agree that a good Golden or Lab is a wonderful match for what you're looking for.
A rough collie (think Lassie) or smooth collie (the shorter coat version of the breed) might be a better fit for your criteria than a sheltie (they're in the right size range too). Like any herder they can be on the sensitive side, but are generally less intense that way than borders or aussies. As with any breed known for their intelligence, you do have to be on top of keeping their brain active and engaged with training, games, puzzle toys, etc. as well as meeting their physical needs, but collies have a reputation as a wonderful family dog for a reason.
With any dog you choose, if you're going with a puppy make sure you vet the breeder thoroughly, and that their breeding goals align with what you want in a dog. For example, you could get a German Shepherd from an excellent breeder who does everything right, but who focuses on breeding dogs for police work and bite sports - these lines tend to be much more intense and high energy than what you want! So find someone focusing on producing dogs who are great family companions with excellent genetic health (this involves tests and screening way beyond 'the vet said my dogs are healthy') and stable temperaments. Definitely meet at least the mother of the litter, and the father or any other close relations if possible to get the best idea of what your puppy is likely to be like as an adult - no one can guarantee a puppy will grow up to be exactly like their parents, but you can definitely rule out a litter if the mom seems anxious, excessively shy, stressed out, etc.
Adult dogs, whether you rescue or buy an older dog from a breeder (sometimes a pup gets returned, or doesn't work out for what the breeder had planned, or gets retired from breeding and needs a home), are a little more 'what you see is what you get', but don't be afraid to ask questions about the dog's history, behaviors, and quirks. Some rescues/shelters allow you to take a dog home on a trial basis, where you can return them within a certain time frame if they don't mesh with your family.