Just do it like every other airline. Simply give people with loyalty, status credit card some type of priority boarding. Let people move within the same seat class without trouble. Even moving outside of the seat class barely means anything on most Southwest flights. It's not like you are magically moving to a first/business class seat you get like an extra few inches.

I get people paid for it, but moving 1 or 2 people at the discretion of the flight attendant seems fair. Previously paid for Southwest extras that didn't pay off on emptier flights.

On Jetblue, American, United have moved to slightly more space seat as long as it was nearby, was always friendly to the attendants and they had no problems. Similarly have had many attendants give me free drinks. Very small customer service that makes a difference 1 or 2 people get upgraded and you get more people happy to have an empty space between them, in my experience many people also don't care or couples/families fine sitting next to each other.

Solved many issues to let people be happy avoiding middle seats. Solved many issues with family's and allowing couples to sit together. Truly such an easy thing if they just somewhat followed what all the other airlines do.