I'm amazed you had such poor results from solar. I'm inclined to think you had it set up wrong, or it was too small for the pool, or you had insufficient sunlight.

Even in limited conditions you should get far better results. My folks had an 18x33 AG in their backyard in upstate NY. I put in a 4x20 and a 4x10 panel and they only got 6 hours of direct sun a day on either the panels or the pool. But the solar cover wasn't just on at night--it was on whenever the pool wasn't being used. We got a 6 degree increase in water temp and maintained it all summer.

I pushed the water through as fast as the system could stand--it's all about heat transfer, not temp.