On my home-built moped i am using a similar belt drive and the large pulley is raw aluminum (without anodizing).
I never heard such squeeking noise, only the high pitch whine you can hear in the first vid from Allex (bike inside a room with the wheel off the ground).
But almost since the beginning i am using this stuff:
https://www.dryfluids-shop.com/epages/6 ... 011%203%22
I did use it because i wanted to see if the no-load consumption could be reduced when adding lubricant to a belt drive, and not because of the noise.
It was just a few watts less so not worth to mention, but maybe this dry fluid is the reason that i don't have issues with such ugly noise.
As it is dry and not sticky it doesn't collect dust or debries (the liquid will evaporate after you apply it and PTFE particles will remain for lubrication).
The blue color anti friction coating on the gates the belt has gone after a few 100km and the dry fluid makes it look grey now (probably from aluminum particles).
Joachim, would you give it a try before you put back the chain on your SurRon?