My Berners have all been very different - the girls always more high energy than my males. Picking the dog should be more about matching a dog specifically to Darcy and her personality than focusing on a breed per se.
Many of us in Berners also have goldens - goldens vary tremendously (so overbred how can they not) so I am not talking about field dogs, but rather a mellower golden. My golden is backyard bred rescue that I doubt had any active working aptitude in her lines, although she is an intense tracker. With that said, my golden struggles to keep up with my Berner...who is very high energy and sprints hard and jumps everything she can. So depends on your Berner.
If you speak to behaviorists and good trainers, you will find they always recommend to never deviate by more than 25 lbs. between dogs. More than this during play is dangerous and the smaller dogs often get hurt - acutely or over time and have arthritis problems later. Dogs are also more defensive and naturally protective of themselves if the disparity is too great.
If Darcy is truly a couch potato and doesn't wrestle play, a small dog may be fine.
The goal is to match energy levels and exercise needs - this is what your parents will find the most challenging if the dogs are not on the same page - other attributes are less important. It is a struggle to be out walking one dog three times a day, when the other can't or has no interest for example.
If you go to a purebred rescue group, they will evaluate Darcy and match accordingly - this is the greatest benefit since they are experts in their chosen breed. Golden Retriever rescue for example should be only discussing those dogs that match well. If you went to Italian Greyhound Rescue for example, they would never give you a dog with a Berner (too small, too fragile, poor fit). Shelters on the other hand cant help you at this level and don't have this expertise, but Darcy may still find a dog there she likes. At the end of the day, Darcy will show you.
Some sporting dogs, depending on activity level and other working dogs, depending on same are your best bet. Avoid Herding dogs and terriers.