A few things to consider:
For Z-rated tires, that mileage is acceptable. Z rated are way softer and will wear faster in all ways. If you don't rotate them at LEAST every 10K, you can expect to get uneven wear and therefore shimmy, pull etc. Your driving habits and inflation makes a difference too. If you drive hard = fast wear. Depending on the inflation psi, your wear will further accelerate on the corners or the flat, causing the pull and shake.
On the other hand, you father's Olds with 50K radials and Sunday driving could never rotate tires and probably would be OK, unless front wheel drive, which by the way, really eats up your steer tires 2x faster than the rears.
On your new tires, check regularly the tread groove depths across each tire (when you get gas). Compare outsides to middle depth as well as forward tires depth to rears. That's the best indication of what psi is ideal for your setup. A little shallow on the edges means put more psi in them. Opposite for shallow middle tread. Even if you go with the recommend psi, it may not suit your driving conditions. Wear tells all. If you go 10k and don't see much difference in wear, you're dialed in. You can buy a tread depth guage for a couple $ and that really is a good investment. You can measure and detect /32 inch differences much quicker(mileage wise, and therefore correct sooner) than if you had to wait and use the finger method to notice an 1/8 difference.
I used to have a hauling co. and used Dualie trucks, trailers etc. I had many an experience with all types of tires and loads. PSI is VERY important to the life of your tires like they say. WIth proper rotation and psi maintenance, I got 80k+ out of my stock tires for and F350 dualie as well as 14-ply tires for my trailer granted they are not Z's but still way better than rated. That's with carrying sometimes up to 4500 pounds in the bed most of the time too!!
Balancing is a little over-rated. They'll do it when you buy them but if you rotate ocasionally and your psi and alignment is decent, you should not need to balance them. Save you some $ later.
I bet you didn't expect a novel did you. Good luck on the continentals.