Firstly, hats off to you for trying to properly diagnose the problem and trying everything that you did. Hope you find the solution soon. Some random suggestions if you haven’t already tried - clean the battery contacts (I’m not sure of the best method to do so but I’m sure you can find something online), check to see if the problem exists in different screen refresh rates, turn off auto brightness if its on.
How are you measuring your speeds? I think cloudflare speed tests were more accurate for me then ookla, but in the end downloading a large file over usenet gives me the best picture
Edit- and that made me realise my ssd was a bottleneck, replacing that helped me go from 500-600 to about 900-950 on my gigabit connection