So I set it to turn off at 65 or sunset. Seems to work.
You might get into trouble at some point with that. It
will work, but maybe not as effectively. If the pool is at a comfy 85°, and the air drops to 66°, your "heater" will still be running and will actually end up cooling the pool, until the air hits 65. Heating a pool with solar can be additive over a few days. By that I mean, maybe that cooling period won't affect your swim that day, because you got out early, no big deal. But it could affect the next day or two, as maybe the pool will not get as warm as it would have because the heater has to make up for that loss.
You don't necessarily "start fresh" each day. The heater starts adding degrees to whatever the temp of the pool is in the morning. It'll be warmer sooner, and/or end up warmer later, if the pool was 75° in the morning as opposed to 65°.
That's another feature of a solar controller. It not only turns on the flow through the panels only when they can actually make the pool warmer, it also turns
off the flow before sending water through the panels could make the pool cooler. Additionally, the controller modifies the pump speed, too (if you have a VS pump), such that the pump ramps up only when the solar heater is engaged, and then runs slower when the panels are not heating the pool. So the controller maintains a warmer pool using as little electricity as possible. (Pardon if I already made that point.)