Something else you might try in the winter is getting a livestock watering trough. Farm and Fleet sells sizes from 40 or 50 gallons all the way up to 300 gallons. They're made of a very heavy duty plastic designed to be outdoors all winter.
In my opinion much of the "classic" betta aggression we see is not inherent to the fish at all but it's a result of what humans have done to them. They manage to coexist with each other and other fish species in their native habitat. So I think the key is to move as far away from the betta-in-a-bowl and as close to their natural habitat as possible.
Annihilating potential mates is a very poor reproductive strategy. From a survival standpoint, aggression really only makes sense if you're a carnivorous predator or if you're trying to defend against those predators. Otherwise it's anti-survival.