This pool has solar but as it's winter here in Florida I don't think it's helping very much. Ordinarily, I would suggest putting in a heat pump coupled with a solar cover. Because this is an infinity edge pool, the cover couldn't be difficult. You can see in the photo the duck looks like he wants to end it all!
Do you think propane would be a good option? Depending how quickly it could heat this 16K gallon pool with it's new 3hp variable speed pump I'm thinking a 400,000 btu propane heater could be turned on in the morning and have the pool ready for swimming in a few hours. Obviously permits, installation cost and propane cost would have to be a factor. Swimming at this pool is not done often.