Looking good! A few thoughts from things I've read or learned:
(3 and 4) I discovered that racing drone motors work the best. Not sure what you're using, but racing drones tend to have very powerful and weight-optimized motors.
They're diatone mamba toka 1606s and weigh ~16g, or about half that of a standard miniquad motor. I'm just not sure if the rotor I built is better matched by a bigger motor. One problem I kept running into is the t-mount pulley on the 1606 melting. I made those "fan pulleys" but they don't work very well... maybe having the motors hidden from rotor wash is not a great idea.
How long have you continuously run yours at 100W power?
And then I'd recommend taking out that center piece that connects all the pitching linkages. It's back a few pages, but I tried that a year or so ago and it completely messed up the pitching and reduced thrust.
The piece in the middle is TPU and supposed to be flexible in the directions required. You think that's causing problems? It seemed like it was working pretty well. How much torque is there on the control input when yours is running? On mine it was pretty significant, but, nothing servos couldn't handle.