Just bought a 2016 F250 w/28,000 miles on it.
Should I splurge for the premium ESP at $2,000?

Sounds like cheap insurance to me with the cost of maintenance on an F250 if something goes wrong.

Or I put my money towards other "improvements" to increase reliability.

Any other advice to keep this truck running strong besides good fuel (and additive) along with regular oil/fuel filter changes?

Yep, 6.7.

That kit seems like a wise choice based on all I've read.

It is.

Case in point, my neighbor and I both run DPKs on our trucks and his pump is going out (we confirmed it last night by cutting the DPK filter and looking at the fuel in the filter).

The DPK saved his fuel system - sucks he needs to buy a new pump but way better than the alternative

Put it on a diet.

Agreed, weight loss on mine was definitely worth it.

Did you do the DPK yourself?

What's the term of the ESP plan. The ones I have purchased have been lesser of miles or years from original purchase date. Maybe it's different for a used purchase.

I would not buy a dealer service product.

I did not, had a shop do it while they were doing a few other things.

Could get this guy to help

3 year/60,000 (additional) miles

I think the repair history on the vehicle prior to your purchase would be a large factor when placing your "bet" with an extened warranty. You are betting something will go wrong while the company that issues it is betting something won't. I bought a F150 new off the lot in 2019 and I had to take it into the dealer for some odd things during the warranty. I opted to get premium care for 5 years past the factory warranty with $0 deductible. In the last year it has paid for itself five times over.

If you are going to buy extended warranty, be sure to shop Flood Ford. It is legitimate and by far the cheapest option I found several years ago.