So this is just to let you all know I FINALLY solved the problem! The motors I was using were the issue.
Having never built a quad before, I followed the advice of an online blogpost, and bought the SunnySky X4108S-17 380KV motors. However, their amp draw at full throttle was suspiciously low, and I was never able to get enough thrust to even get off the ground. The drone would just dance around lazily.
A word of caution: do not take your chances with this motor! Trust me, there's probably a reason SunnySky discontinued it's production.
Swapping out these motors for the GARTT 4112 400KV motors instead, the difference, even at low throttle was immediately apparent. Check out the thrust test I did below:
Having bought only one initially, I took the plunge and shipped in the other 3 motors (choosing the wrong motors is a very costly endeavor btw, wasted over $150 on bad motors) and I finally took flight at 1am on September 29th, 11 months after starting the project! With an AUW of 2300g (no payload), the drone starts to take off at just 40% throttle, a clear indication of the thrust potential.
To sum it up, if you ever have problems with lack of thrust, always check the amp draw of at least one of the motors, ESCs will rarely be the issue, as they only give the motor what it requests. Also, ESC issues normally manifest in other ways, like a stuttering motor.
I'll post a full flight video soon, but for now I'd just like to thank you all for the support and advice with this issue, and I do hope this information will be of use to someone