I lived in NC back in the 70's and early 80's. You start with a five month swimming season without heating-May 1 to Sept 30. And it's pretty sunny during those months. But April and October tend to be rainy so solar would be a good choice during the five months to offset early and late season chilling.

Everyone I've known with a gas heater finds them dreadfully expensive to run, and few end up using them much.

Heat pumps have a bad rep from being poor HVAC alternatives to separate furnace-a/c units...but it doesn't really follow because when you use one for your pool, you are using it in its "sweet spot" where it is most efficient.

VS vs 2spd pumps is a balance of choices. VS pumps are more efficient, but by how much? Plus if you are subject to spikes and brown-outs, as well as lightning, you MUST have surge protection for VS. 2spds cost less, and, on low speed are very efficient, reliable, and easier to repair.

Frankly, I did not know 40' long FG pools are even made.