Things that will improve your dog's off leash skill that aren't strict obedience training:
Regular settle practice in a variety of environments (very different than down-stays)
Scatter feeding (even if it's only once or twice a week)
Giving more freedom on a regular basis
Letting them sniff more often (yes, even the dogs who get so caught up in scent trails that they loose their mind. If sniffing and tracking is a rarity then it will be of higher and higher value the less they get to do it and they will seek it out more and more if they aren't getting enough stimulation)
Regularly rewarding them for engagement
Tattle training (engage with you for a reward whenever they see a dog/wildlife/bicycle/whatever they may find distracting)
Don't rely so much on the leash or any specific tools for communication. Dogs are naturally much more attuned to movement and noise cues so learn how to communicate that way
Regular time on longlines and safe off leash spaces without much expectation
Switch up the gear they walk in occasionally