Gotta be a slippung belt.

Now, I can’t remember the vehicle, but well over 20 years ago I worked on one the the light would come On above about 1500 engine rpm. It was the alternator. I have little clue why, but it would not charge above that speed.

If i read right, you have changed the alternator. Well, I changed the one on my SE in June of 2015. In August, it failed. NAPA replaces it free. Rebuilt alternators fail.