THE cheapest way to keep the pond warm if you do not have a natural source is air spice heat pump, but that is based on running costs, they are expensive to buy and install. I am in the uk and my tropical pond outdoor in the fish house was unheated and dropped to around 60 degrees in winter. Providing it happens gradually and the fish grow up with it happening I found that most could cope. Only the jelly cat could not and that’s when it dropped to 58.
Thick >2” insulation is a big thing, and not just underneath but all around. And covering the top helps with a greenhouse style covering to not only keep the heat in but to help build heat very slightly on sunny days.
Multiple 300w heaters set to 24 degrees can help but to know how many is a try it and see type of situation. I certainly would not have them set to 32 as they will never be off and the fish no not need to be anywhere like 32 in winter.