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Check how hot the auxillary battery terminal gets...mine would get hot and cause voltage drops until I replaced it...previous owners thought it was just going through alternators. I had one of the zinc ones (and one with the bottom threads stripped so it was had a couple of washers for spacers and what appeared to be a Delco radio post nut as well) and replaced it with brass...common issue on the B bodies too, and it's the same process and part number for replacement. Might not apply on a 2000, though... Replacing the Auxiliary Battery Post | Page 3 | Chevy Impala SS Forum

Please do. That slight voltage sag (dimming/fan speed drop) that took place above when the AC compressor kicked in is 100% normal operation for these cars in my view, and its just a byproduct of how the charging system is programmed to function. It can happen whenever a large electrical load comes on line at low RPM, and It is definitely visible in the headlights when they are on. My 2000 Regal did this all the time, and the factory alternator and the 6 year old battery were still fully functional when I got rid of that car in early 2015.

I'm going to be honest, I don't see an issue there. It cranked just fine, and the dip sounded like the AC compressor kicking on. The slight electrical dip is likely just a product of a slight lag between a large electrical load kicking in and the duty cycle of the alternator being increased adequately to compensate.

HERE'S MY CAR AFTER ALL THE VISUAL UPGRADES

That's the one. It looks like you took care of it with your wiring upgrade though.