Valves usually break due to age, fatigue1, and a combination of environmental2 conditions to which they are exposed. Exhaust valves are repeatedly loaded and unloaded as they open and close, they can fatigue earlier than normal if the environmental conditions to which they are exposed to are severe enough. These conditions can be a combination of temperature, stress and a corrosive environment. If any of these conditions are excessive, the valve will break its weakest point.

(1) In materials science, fatigue is the weakening of a material caused by repeatedly applied loads. It is the progressive and localized structural damage that occurs when a material is subjected to cyclic loading. The nominal maximum stress values that cause such damage may be much less than the strength of the material typically quoted as the ultimate tensile stress limit, or the yield stress limit.

(2) Deposits formed on an exhaust valve can be due to the reaction of fuel-borne or airborne contaminants and lubricating oil created during the combustion process as well as the reaction of combustion products with valve materials themselves (chemical). Build-up of deposits on the valve tends to have an insulating effect that can limit critical valve metal cooling (heat transfer) and can make the valve run much hotter than normal which can lead to hot corrosion or exfoliation of the valve stem material.

We know from experience by many on this forum who have suffered timing belt failures just how tough the valves themselves are as many have made severe contact with the top of a piston hitting it hard enough to break or crack rocker arms without sustaining any valve failures in the process. With this history, I believe it rules out valve failure simply caused by contact?

So this leaves failure by some other means or process. Manufacturing defects have been discussed but not entirely ruled out. I think valve stem cracking was ruled out as well in some of the past tests reported? Stated failures are clean breaks between the valve stem and valve mushroom head?
I still believe that the high soot loading these exhaust valves are exposed to from the EGR system may be a contributing factor in their failure, be it chemical as described above or mechanical from abrasives created from the soot and combustion process and possible exfoliation of the valve stem material, or a combination of both.

I think what i'm asking is.
Answers in Blue Text
Do I dis-assemble the head and take everything to the machine shop with new parts. Asking for an R&R on the head while installing new exhaust valves.
OR can I dis-assemble the head and put new exhaust and old intake valves in myself and use lapping compound on them?
If you don't suspect a cracked head, no need to take to machine shop, you can pull the exhaust valves, lap in the new ones yourself using lapping compound.
If you don't see any evidence of a leaking intake valve, I would leave them alone. No need to mess with them.
And this head by design is not supposed to be milled, very little extra metal on the combustion chamber. The Factory Service Manual actually states NOT to machine the head.

I'm guessing all the retaining hardware used on the cams should be replaced with new.
Not sure what hardware you are referring to?

Can the valve keepers and spring retainers be reused?
If the hardware is not damaged, reuse it, it has already stood the test of time, so you know it is good. No need to replace with new!

and not replacing the valve seals would be pretty stupid.
On this engine, the seals don't serve much purpose since the intake never goes under a vacuum. Seals are to keep oil out, not combustion in. Actually a little oil on the guides is a good thing.

So I'm looking at this set of parts from IDparts.

8x exhaust valves.
valve stem seal set (don't need)
Seal, plugs and snap rings for the cams. The super service kit seems to option the plugs but claim they are out of stock. but in stock for single plugs.

the super service kit with rockers and ARP head studs add intake and exhaust gaskets and hardware.
or head install kit with rockers and the timing belt kit.

Its unclear whether the super service kit or timing belt kit come with serp belt idler pulleys after adding the serp belt tensioner as an option.
The Timing Belt Kit comes with the 2 idler pulleys


Am I missing anything?
*Since both kits list a Serpentine Belt, I would call them and get them to back one of the belts out to save a few dollars.
or simply buy the timing belt kit and then the serpentine belt tensioner and 2 idler pulleys separate.