Some more mpgs will show up after you get a few thousand miles on it, no doubt
I like to keep tire pressure within 2 psi of the tire's rated max in the summer. Not as comfy of a ride and too much pressure can hurt brake performance and cause uneven tire wear so don't go crazy but a little more than what the average dealer sends it out the door with usually helps.
Driving that many long, consistent highway trips you may get a teeny tiny bump from full synthetic oil, probably not more than 0.2 MPG compared against the worst cheapest oil out there but little things add up.
Actual mods well, first off did you somehow get a manual transmission car with cruise control? I don't know that I'd be willing to make that long of a drive that often without cruise
I'd be hunting for the rostra kit because on my good days I can do way better than any cruise control units but on bad days... my gas foot won't stay steady and lock into position and I end up wandering between speeds which is generally not efficient.
Might get something measurable from some cheap lowering springs but not sure the increased ride harshness will be worth it, even as cheap as some springs are it will take a lot of miles before you make that money back. I have a way of cheating that system but its so highly not-recommended that I dare not even mention it
(No, not talking about cutting springs, don't do that!)
What really might help and also be cheap would be some budget aero mods. These things are really popular over at geometroforum.com where they squeeze every last drop of gas mileage out of already very efficient old cars. Partial air dam blocks, using garden skirting to create a lower front valance/lip, lightweight sheet metal or plastic panels under-body, anything to streamline the flow of air around the car. The hardcore will pop off their windshield wiper arms and remove one or both of their mirrors even. Note, due to different cowling design I would not expect anything could be gained from removing the wiper arms on a versa, don't bother. Also not sure how you could remove a mirror without it looking weird, but when you think about it you really don't need
the passenger mirror much do you?
I think underbody aero panels would be a good cheap starting point. Many other cars (Honda) come with them stock and they do seem to help reduce turbulence under the car. Its cheap to go into a home improvement store and buy a 10 foot long roll of roof flashing tin 10 inches wide and a 200 back of zip ties, some tin snips and go nuts! Have to be careful though, you don't want to screw up airflow in the engine compartment, you don't want to add crap that hits moving parts of the suspension, you don't want to trap the exhaust between some tin and the bottom of the car, you don't want to end up with a bunch of tin flapping around under the car at speed making a racket... having a bending brake to add some rigid lines would help as would bolts or rivets in place of zip ties... in theory there is probably something to be gained but the hours of screwing around making an ugly conglomeration under the car... not usually worth it on something new and nice. What we do to ancient 3 cylinder beaters on GMF is a different ballgame, those cars never go anywhere for service except home. (Could you imagine rolling into the dealership for service with a couple rolls of flashing and some garden skirting under your car?
Sorry I guess none of this is super useful in your case.
Taller skinny tires would help lower the final drive ratio which would almost certainly help as long as they weren't too much heavier than stock, but not enough to justify getting rid of good tires that are not worn out. Just something to think of when you DO wear out a set of tires.
I'm just about done spewing my 2 cents now... generally speaking nissan already made this car very efficient for what it is and they did not leave much to improve on. If you have the time and resources to make a nice front engine aero panel (not a tin conglomeration) you might gain something, the rest of the underside is smooth enough really. If you have 1 inch taller tires already sitting on lightweight rims with the same bolt pattern then by all means, lower your highway RPMs a bit. Delete the passenger mirror and roof antenna if you want to get .0075 more mpg...
But essentially come to the conclusion that the nissan engineers did not really leave any low-hanging fruit on this car for you to pick at