I’ve made ever moped, every moped I’ve ever used to have great brakes.
Few things to do.
1. Sipe the pads (dremel slots in each shoe perpendicular to the shoe or on an angle) scuff up shoe top after with sandpaper
2. Lube connection points and the paddle that moves the shoes. Make sure it moves easily
3. Scuff the drum with sandpaper then clean well.
4. If the hand levers aren’t returning well, add springs on the cables between the stops
5. If the cable is too thin it will stretch and not grab well. Use at least 1.5-2mm inner cables.
6. Adjust cables and sometimes you must cut a lead end off and add a Knarp to get things closer
7. Engage the brake when tightening the axle to center the whole shebang
This is all I do and my brakes are kick ass. Even on my tiny 16in 5 stars. Sipes are the most amazing thing. I do them to all brakes and clutches (edited)