Yes I’m positive the glow plugs are actually turning on because they are still pretty warm after 5 minutes of turning them on and the switch has no measurable amperage going thru it, just completes the ground from the gp controller. And the truck will start immediately after I shut it off but not 10 mins after it’s shut off.
First of all I'd like to say that it sounds like you've done some good work trying to identify the problem. That certainly helps us all out in trying to help you!
From what you're saying you may have two different problems. Starter speed/glow plug health, and IP health.
Good compression means there are only a few issues left to sort:
1. IP health. It sounds like you have the classic hot-start issue with our IPs. Next time you have the engine hot, shut it down and wait 10-30 mins to try and start. When it won't, take a water bottle of ROOM TEMP water and pour it over the back(towards the cab) of the IP. Then proceed to start again. If it works you have a tired IP and need to start saving for a new one. Buy only from the good rebuilders!
2. Starter speed. You said you changed starters, and have about 300 rpm starting speed. That sounds really fast... can you count the revolutions? If it's too fast to count then you're good. As an example, due to my fast starter my engine will start without glow plugs on an 85*+ day. And I've never had a compression test in 15 years.
3. Glow plugs. Unfortunately OHM readings don't work very well with our GPs. They can work, but it's not very consistent. How long do you hold the button? Give it 10 seconds if you're not already. Another thing is that the yellow power wires often melt at the connector by the HVAC blower. Check that out for bad connections, that could be the problem, too.
4. Timing affects starting and other things. Too advanced hurts it. Too retarded helps it.
I know you replaced the starter, but are your batteries and cables in good condition? The speed that you spin these engines is a big part of starting them, especially when cold. Lastly, when was the last time you checked timing?