Bach-On!!!
You could be right in hotter climates but where I am the underground water temp about 4 to 5 foot down, (watertable), is 46°F (8°C) give or take. What I'm using now is pretty good (120 litre stainless tank buried in the surface under a mobile home), but it is still affected too much by the general ambient air temp. Underground in the water table it will basically be at the yearly average temp where you are, unless you go very deep as you said. I'm sure the yearly average in Florida is more like 62°F (16°C)
I've not checked the temp of my water supply but I have an artisan well that goes down 180 foot into the pure underground river. It comes out and is filtered through an Iron & manganese filter in the garage and this is always condensation city in the summer.
Best thing for me to do is stop waffling and get on a do it
like said, I do have a old JCB so digging the hole is not an issue. Just got to drill and tap the inlet and outlet fittings on the Propane cylinder now.
I'll bring this topic back with the results later
