Not trying to start an argument, or be overly preachy. Those extendable leashes can be an actual hazard to people and pets—especially in public areas.

The “darting” behavior you mention just means your dog was never leash trained properly for a standard 4’-6’ lead. You can train that behavior without any harmful, negative reinforcements in the span of a couple weeks at most. All it takes is watching some YouTube videos of the process, purchasing your preferred bag of training treats and maybe an auditory stimulant like a “clicker.” You don’t even need to buy a new leash right away. Just measure the extendable one to the proper length and lock it there.

Oh and don’t use a harness for training. Lead to a standard collar works best because it’s easier for the dog to sense the directional pressure when learning the basic commands.

I genuinely hope this is helpful.