I just tested the positive high-side switch portion of the motor controller and everything seems to be working as intended. The section including all parts involved is circled in a bold blue line to indicate the portion I just tested successfully.
One issue I'm having though is that the drain of the A09T attaches to the 100ohm resistor tightly and is a weak point that broke off twice now. Hardly any wiggling at all on the arduino input line and ground line leading into the A09T mosfet causes the drain solder attachment to break off. I am wanting to glue it all down onto the mosfet but I'm supposed to tape the heatsink on under all this stuff so I don't think I should glue it down. I need some kind of backing sheet to glue things off onto (where a PCB normally does this job). Which will provide much needed strain relief at all attachment points. I guess I'm learning the hard way why PCBs are used in general. Without a flat backing plate or substrate of some sort the attachment points between components are vulnerable to flex and breakage super easily. This surprises me.
To perform the test I used one lab power supply set to 20v and one set to 8.07v and used a 18650 lithium battery as the 4.12v to simulate the arduino output pins. I carefully electrically isolated all the metal lines with packing tape for now to ensure no short circuits and then I connected the lab power supply pins to the correct locations with alligator clips. Finally I connected the 18650 lithium battery 4.12v to simulate the arduino turning on the A09T mosfet - I did this using the two nickel strips for this portion joined to the battery with neodymium magnets. If I had a 3rd power supply I could get 5v off of I'd have done that but I didn't have one in arms reach so the battery it was. The LED came on and I tested the output line to the motor was indeed 8.07v. I then disconnected the + side of the battery and verified the line going to the motor was 0V. It was - although if I kept the multimeter on that line longer I noticed it would creep up to like 3.4v but something similar happened on my last test run and chatgpt said this was like parasitic capacitance involving the multimeter or something and nothing to worry about. The main thing is it would START at 0v when I first connected and then rise up to 3v or w/e over time on the multimeter screen and this behavior was ok last time so meh. We're good I think.
Where to go from here then? Well I'd say I make the other (lowside) portion of the half bridge and then test the full half bridge to ensure it's all working. I think then my design is validated enough to move into diy flex pcb for some of these portions that are on the layer that goes onto the main beefy mosfets.