Hard start after it sits a couple days?

Hello, it sounds like you have a good idea of what you are doing. Do you have a scan tool that can monitor live data PIDs? Also, this truck has 2 fuel pumps, the one in the fuel tank and then there's a balance fuel pump on the inside of the left frame rail. Fuel pressure spec is 56-62, you might be having a fuel volume issue. Have you done a fuel filter in the last 20k miles or so? With this size engine and at a cold start, one concern might be fuel volume, I know your pressure is staying okay.

If you can pull up live data, I would graph the front Oxygen sensors and the rear ones, see if the signals look like after its warmed up and running. If the rear oxygen sensors mirror the front sensors, Id suspect a Catalytic Converter at that point. Another thing to look at is the Long Term and Short-Term Fuel trims, to get an idea of how much fuel correction is happening with the ECM. If you notice positive fuel trim numbers at idle and the numbers, come down after you start to drive and the throttle plates are open, that will point to a vacuum leak. Fuel trim numbers can tell you a lot about the running condition of the engine. So can the oxygen sensor readings, a wide-open throttle test, the oxygen sensors should go rich, if they lean out (low voltage) then suspect a fueling issue.
Do you have good strong spark too. There's a few things to check.
One other thing is checking to see if the throttle plates have any carbon build up on them.
Something I forgot to mention, the Fuel Trim with Chevy/GM can also be called the Block Learn. That would be a long-term fuel trim in memory.

Here's a couple TSBs I came across as well:

https://www.2carpros.com/articles/engine-cranks-excessively-before-starting

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge