I have an old aeromotor windmill on a water well and I was wondering if there is an easy way to have it fill a tank and then automatically shot off when the tub/trough is full? There’s a large dug out earth tank near by but I feel like it might be a waste to keep it full/pumping into that?

No

How big is the holding tank? I would rig the holding tank with an overflow that runs into the dugout, that way you always at least have X number of gallons in reserve.

Float valve for tank, with a vertical-loop-bypass (T-fitting before the valve) back down to well.

Would I need a well service to do this or is it pretty straight forward?

Satellite image of the windmill, water trough and dugout next to it. Would I run the ground water dry pumping it into the dugout? Or would the water just soak back into the aquifer?

The dug out is for over flow from the tanks. Basically once the tanks are full it empties to the dug out. It’s a simple system. Bottom part of the tank has a lower hole for water to pipe to trough with or with a float, the top part of the tank has a higher hole that pipes to another tank or over flow pound/dug out

Only way to turn off a wind mill is to discount the pump rod that is going up and down when the wind mill turns with the wind. Or chain blades from moving.

So what you are asking is basically no. You can’t auto shut off a wind mill, unless you stop it from turning or disconnect pump rod

Does the windmill work now? They can be a money pit. I speak from very recent experience. You'd be money ahead to convert it to a solar pump now and have them rig it the way you want while they're there. The old dirt tank may need clay or bentonite brought in for it to seal and hold.

It looks like the mill filled a holding tank ("pila"), the holding tank filled a lower round trough, that might of had a float, and you could wire the float up and overflow would go into the dirt tank.

However you decide to go , good on you for resurrecting a water source.

What does the well top/cap look like? Pic?
If there's no place for an opening, then it might not be an option. IE, if drop pipe is as big as well casing.

Also, if you can't divert back down the well, maybe divert to a food plot?

That’s a good point. How much does a bentonite seal cost? Sounds like a solar pump may be the way to go in the long run. Are the solar pumps expensive?

Depends on how good and deep the well is.

I’ve replace 6/8 with solar. I have two windmills left. The last two only run 5 months of the year though.