Yep, 2500 was the recommendation (also for the 6.2L diesel Blazer I once owned) for short hoppers like you. It was 5K miles for mostly freeway driving. Those recommendations are dated to the era and oils have improved since.
In the 10 years I had my 6.2L, I ran 4K intervals in mixed driving but that was before I got oil religion. I would have verified that with UOAs today. My instinct is to say 4K is OK, even in your environment, because I still own an '86 F250 diesel (bought almost new back then) and I have verified 4-6K with that engine in mixed driving. The 6.9 and the 6.2/6.5 are both old school IDI engines with nearly identical fuel systems. My 6.9 was turbocharged with a Banks kit right after I got it, as well.
My advice to you would be to improve your filtration, try 4K OCIs and UOA. Go to a lab that can measure soot (that would eliminate Blackstone), such as Wearheck, Oil Analyzers, Polaris, etc. Soot will be your condemnation point. Some of the better filtering filters out there are the Pure 1, the Royal Purple, Mobil 1, Amsoil. Unless you get into a highly efficient bypass system (with under 10 micron filtration) you won't see huge changes but better filtration will keep the soot levels lower over time.
When you UOA, look first at viscosity and soot. Ideally you will be under or around 1 percent, but you are safe to as much as 2 percent or so (according to some experts more is safe but.... I hate to see it higher than 2 percent base on my own studies and I find I can go a long way before it gets that high).
Viscosity will tell you if the engine sheared the oil. My 6.9L cannot shear Rotella before the soot load builds up first, so I doubt you have to worry about that unless you are getting fuel into the crankcase.
I'll toss in an unsolicited viscosity recommendation. In your use, you should be using the Rotella 10W30 IMO (I do and did in both the 6.2 and the 6.9L). It's plenty robust but will flow better in those situations where the oil is not fully warmed up (which take 15 miles of driving at least). Perfect for a short hopper and it's recommended in the manual. My 6.2 manual says straight 30 is preferred but that was in the era when 10W30 grades tended to shear a lot more. If you are using 15W40, you are fine, but I think you might see a slight improvement in MPG with the 10W30. Cold starting will likely improve too. Cold starts were the big bugaboo with my 6.2L and every RPM more aI could spin it up on a cold morning was beneficial.
Unless the wear metals are off the charts, don't get roped into sweaty palms over a few PPM one way or another. The lab will call out any dangerous wear metals. In the first UOA, the amount doesn't tell you much because every engine in a set use and maintenance schedule will produce pretty regular results that may be different than another guy in a completely different situation. Since you are just starting, you don't know what "normal" is for your engine. Wear metals get to be a braggadocius element here but most times people are comparing apples to grapes.
