This will hardly be complete, but... Things I see.
It's not an ESC setup issue, but I do like to use the MOT_SPIN_ARMED and MOT_SPIN_MIN parameters to avoid "soft start" features in the ESCs. You have them zeroed. This is also why they start so late as you raise the throttle from bottom in STABILIZE.
This behavior is odd.
C3IN, THI, and THO are synchronized as expected. RCOUT 1 through 4 clearly are late to the game.
I also like to reduce my DISARM_DELAY from 10 to 5ish for auto landing/disarm. This will not influence the throttle issue you are seeing, it would just "idle" for less time on ground. The way you have the MOT parameters setup, you don't even see the "idle" after arming, or before disarming.
After seeing your non-standard MOT setup, I think the previous suggestion to go back to stock parameters and bench test is warranted.