One of the most intractable problems I found with my E-maxs - probably any rear-drum brake equipped, Chinese hub motor e-scooter - is the awful brake squeal on some of them. I tried a number of things like sanding the drum and shoes, anti-squeal compound on the shoes, with only temporary, if any relief.
But finally, I thought up a solution. The brake squeal is just a harmonic that sets up at the natural vibratory frequency of the brake linings under the friction with the drum. So, by introducing a gap in the shoe it keeps the harmonic from setting up. So, I cut a transverse slot across the linings, at the mid-point of the linings with a saw, the full depth of the lining, and a very small amount into the shoe, and beveled the slot with a file. I reinstalled the shoes, and voila, no more squeal.
It only took 3/1/2 years to figure it out!
Considering their Chinese origin, the lining material may be asbestos containing - especially in the US where asbestos-containing materials have not ever been legally banned, so do this outdoors and wet the shoe and saw.