How to increase efficiency of DIY heated pool?

Hmm. I would set up a few car radiators in parallel inside the grill. They’re basically designed to transfer heat. You would just be using them in the other direction.

Increase efficiency by using smaller pool.

Also increase efficiency by cooking steaks while relaxing.

Use a wood burning stove with a heat exchanger from a combi boiler. Cool water leaves the base of the pool and is brought to the heat exchanger within the stove. Warm water is brought to the top of the pool.

Try better tube insulation and also there a big gap in the grill. Try covering it. A bigger copper heating coil should be also in consideration, or make two heating coils. Or ditch out the grill and make a large coil and put wood inside. No pump required for smaller pools.. Probably You would end up whit multiple insulated copper coils. Stacked car radiators should also do the trick like @mholiv said, but You should put the pump on higher speed so they wont start to boil inside. Be sure that the rubber hoses are high temperature proof. Or just put a thick steel barrel into the pool and put wood or coal inside, just make sure to add insulation on bottom so Your pool wont have a burned hole on the bottom.