[oil pan 4]

Second mod should be electric fan conversion or an air dam, when I did the electric conversion I picked up about 2mpg, the air dam netted me 1 to 1.5mph.
Both are fairly easy to do, quick and give good return.

A turbocharger should come after that. Start looking for parts now. When I installed my holset HE351VE turbocharger off a 2007.5-2010 cummins I gained 2 to 2.5 MPG and gained plenty of power. Just don't put a factory sized turbo on it, we consider them too small to go on the chevy 6.2L diesel. Go big with the turbo and don't use the factory piping where you can help it, get what ever banks parts you can. I use a banks exhaust manifold and glad I did.

Don't worry about the vacuum pump, its likely only about a 0.1 MPG improvement as calculated on paper, I did a vacuum pump delete and saw no difference.

Don't worry too much about weight reduction, your truck has close to the same weight and lack of aerodynamics as my suburban, each 100lb that you shave off will only net about 0.1mpg on paper and my towing logs indicate 1mpg loss with the addition of as little as 500lb. You would have to shed hundreds of pounds to get any noticeable improvement.