So I ended up finding a used but perfect condition complete throttle body, upper and lower injectors, and all sensors for an excellent price and started out by swapping out the whole upper injectors and mount. No change.
Over the long weekend I finally got impatient enough to swap my stock exhaust for the Beet I picked up.... annnd.... the issue is gone!!!
So the welds on my original stock headers failed recently and I replaced them with used stock headers in great condition, with the rest of the exhaust original. I'm trying to figure out what caused this issue, and of course after all this trial and error it's the simple answer.
I removed the exhaust valve motor from the stock muffler, but left the valve and spring on... Well I ride daily in all weather conditions, and the exhaust is the one area Kawi chose not to coat in any way... The spring on the valve rusted and the end snapped, allowing the valve to rotate against only the weight of the spring and the force of the air through the muffler rather than the tension it should have holding it open. That's why it was sometimes a little better, or able to rev a little higher or lower before eventually hitting a wall as the pressure closed the valve.
Luckily I only spent about $150 on parts, and I don't plan on selling this bike so I'm good with having backups. I'm going to swap the original fuel pump, injectors, and all parts back as well. This was definitely a reminder to check all aspects of the simple things before moving on to more complicated tests. It also shows that, provided you maintain the bike decently, use distilled water, clean gas, good oil, etc, that you can have a bike with extended maintenance and no issues (that I didn't cause lol).