How to heat up a swimming pool with the sun

I’m trying to figure out how to heat my pool most efficiently using the sun.

Currently, I’ve got 2 black panels (made up of a bunch of black pipes) on the roof that the pool pump cycles the water through as part of the normal cycle through the pool filter. That pump uses 0.75kw of power just to pump the water to the roof which seems extremely inefficient if it’s for heating water.

So I went and bought a very small pump that pumps the water up to the roof and then through a couple of black pipes before pumping it down.

A couple of questions:

1. The water is cold(by feel right now) when it comes back out with both methods. Would it help if I stop the water with the small pump, wait for it to heat up a bit and cycle? I don’t know enough about thermal conductivity to know if heat is transferred more efficiently when water is moving vs standing still as well as the temp of water relative to the plastic pipe being heated by the sun. Edit: ok, I see that the higher the difference in temp, the faster heat flow happens. Still don’t understand the part of it being heating up for bot a long time before going back into pool.
2. Assuming I can repurpose the energy used for the pump (from my solar panels) would it make sense to just rather use a 600w heating element and cycle the water through a geyser / container with an element (don’t want element in pool for safety)?
3. Anything else that’s more efficient?

To add, I’ve got a thick pvc tarp over it (for kids) and I assume because of the air gap between tarp and water most of the heat is not transferred effectively.

The pool is around 15000 liters and ~2.5*5m (not sure of depth).