Can you adjust the table top versus the blade?

Did you join the fence contacting face of your test pieces?

Did the guy use a more stable material like MDO where you used sawn lumber?

Granted I’m still very inexperienced with fine wood working, I have accepted that you have to cheat somewhere if you want to get anything done. We’re not working on metal with machines you need a crane to move onto their own foot thick concrete pad (stable, rigid).

Nobody will ever notice 10thou over more than two feet. It’s probably not practical to try and get more squared than that. Perfect is the enemy of good enough.

Maybe you can FIT the next size up in blade in your saw? Going from a 10 to 12” blade will give you a 20% increase in distance to attempt to see error. Problem is you can’t turn the saw on and make a test cut, so that may be wasted effort. I also question the flatness of large blades.

I am very curious to hear from you experienced guys why my thought process is wrong and how they do it better. Despite my previous statement on perfection, I am very much addicted to it, it makes me feel good to know that I managed to make something far beyond the requirement. I always strive to push myself and my tools beyond what is acceptable. Nobody gives a shit that your 6ftx4ft bookshelf is less than 1/32” out in any dimension, no man can actually see it, but damnit, I know it’s “right” and I’m proud of it.

Remember, 1/64” is 15thou. If I make a 2ft cut that’s less than a sixtyfourth out, I’m sending it.

^^^^ This isn't the answer. You're actually trying to square the fence to the miter slot. Regardless of whether the blade is a 1/4" router bit or a 12" combo blade, if the fence isn't square with the slot the resultant cut won't be square.

OP, if it were me I would drill a hole for a 1/4" countersunk screw through both the base and the fence and then remove the fence temporarily and make the hole a slot so the fence is adjustable. Once you find square, clamp a stop so you can return the fence to the square position, then swing the fence out of the way and apply glue and hold it in place with the screw you installed.

It's also important the slot be square with the blade to prevent binding. This is the first step in setting up a saw and I presume that has already been done.