Hi all,
New to FPV and Soldering

I'm trying to solder motor pads for my 2450KV build using a generic 80W AliExpress kit at 400°C, but the solder won't melt even after I sanded the oxidation on the pad and applied flux. I first had it melting onto the pad somehow while using a conical tip. But now It feels like the board is sucking all the heat out of my iron even when i use a flat tip pressed against the pad, now the pad is turning brownish from all the repeated attempts, and my tip just doesn't want to hold any solder i tried to tin it "properly" by following some tutorials - wrapped the soldering tip with solder and applies flux and let it heat to 400°c no help at all.
Even tried asked AI if i was doing anything wrong but no help at all. 

I have order solder tinner and some brass wool, I'm hoping that's the fix. But my question is am i just stupid and not using the iron properly or does my generic iron just sucks a*s.

Any tips on what i should do next?

​TL;DR:
​The Issue: Solder won't melt on large motor pads even at 400°C; pad is turning brown/burnt.
​The Gear: Generic 80W AliExpress kit (tried both conical and flat tips).
​The Failed Fixes: Sanded the pad, added flux, tried "wrapping" the tip in solder to tin it.
​The Question: Is my iron's thermal recovery too weak for these drone pads, or is my tip just too oxidized to transfer heat?

Seeking advice.