Which one is more energy/fuel efficient to cool the inside of the car: lower thermostat + lower fan speed, or slightly higher thermostat + higher fan speed?

Likely doesn’t matter. Like. At all.

Ac is likely chilled to a given temperature regardless of what the final temp is called for.

The air is cooled by blowing it over a radiator on the cold side of the AC’s compressor, the radiator then absorbs heat from the air (putting it into the coolant.)

As long as you’re not putting more heat into the coolant than the chiller can extract, it’s only going to remove so much heat- more if it’s slower, because it’s in more time, it then it’s spread across less air, if that makes sense.

Regardless, the way temperature is controlled is by mixing it with additional ambient-temp air. (Either cabin air or outside air.)

So technically, it would be most “effecient” to have full cold, but (as already said,) the ac system is effecient enough it basically doesn’t matter.

Going the other way doesn’t matter at all because it uses waste heat from the engine in a bypass loop. That said, if you have a cold car you’ll slow down the engine warming up by turning on the heat. (You can see this when it’s very cold by warming up the engine and watching the temp gauge when you turn on the air.)

EVs are a bit different- heating from cold start is done with electric heaters. Once the car is moving and the motors are hot, they typically use waste heat from there.

I assume there’s little difference between ice and ev in cooling,

I think the more energy efficient will be the second option since you're not cooling to as low of a temperature. The fan speed change doesn't seem likely to be a big energy difference, but not having to cool as much is likely to save significant energy.

I'm thinking higher temp will cause the AC to cycle less, hence less frequent load on the engine and fuel savings

The one detail I haven't seen in the other comments would be to recirculate the air inside the car, so the car doesn't have to cool the hot air from outside, but instead only cool the already cooler air from the inside. To be the most optimal if your car has been in the sun you should make it draw air from the outside until it's a little cooler than the outside then recirculate

theoretically the most fuel efficient would be not having it on at all.

But it really depends on how vehicle refrigerant phase change loops work. I'm not a mechanic nor am i familiar so hopefully someone else can chime in?

There are a few outcomes here depending on the cycle. If it uses some sort of refrigerant flow restriction that is variable, then theoretically the thermostat setting will have an effect. (i would assume this is a case these days) However the effect it will have is not going to be linear, given that its a fluid pump cycle. Though it's powered directly by the engine (most likely?) so that probably won't matter at all, engines are horribly inefficient to begin with.

if it's not using some sort of variable valve to control restriction and thus cooling potential, it's doing some other shenanigans and running the phase change at a set point, though it would have to introduce some sort of external variable there in order to compensate so probably not.

I would probably set the ac to a low temp, and then the fan to a low speed, because it takes advantage of physics better that way (the greater the difference in temperature, the more effectively you can transfer heat away from it) since you're running a low fan speed, you're gonna need a high temperature gradient to make the best use of that. There is also the inverse, but i doubt that would be better tbh.

Though i think realistically, it just won't matter enough for you to bother. Like you'll probably see a bigger difference in usage by taking a different route somewhere, or idling longer. There are a number of other factors that can impact fuel efficiency.

There is some optimal amount of thermostat vs fan speed. Whether you're better with a lower thermostat and higher fan speed or vice versa depends on which side of this optimum you're on. We as random internet strangers have no way to find this.

It depends on a large number of factors. Largely it doesn't matter.

What I personally do is start my car with a low fan speed, let the evaporator cool down, then start cranking up the fan speed so I feel the cooler air. When it's really hot out my AC can't keep up in my car, and blasting the AC essentially guarantees that I'll never feel cold air. I'll just have a cooler ambient temperature, but the air blowing at me (that I'm feeling) is warmer so I feel hotter.