So the way that the self-adjuster works is that when you back-up it ratchets and adjusts the brakes. Because it looks like you have new brake hardware, at least on the RH side, the first thing that I would check to see is if the adjuster arm is properly engaging the star-wheel! I have found on replacement hardware kits, that sometimes the adjuster arm is not formed properly and does not engage the star wheel correctly. What this causes is that the brakes do not self adjust!
How you would check is with the drum off, see if you can hand spin the star wheel. One way it should ratchet on the adjuster arm and you will hear a positive "click-click-click". The other way, it should be impossible to rotate unless you pull the adjuster arm away from the star wheel. In other words, the adjuster arm should prevent you from spinning the star wheel to "back-off" the adjuster, or collapse the star wheel screw so that the linings pull away from the drum. If the adjuster arm does not function in this manner, it will not self-adjust properly.
Also, in your video you ask what that slot in the drum is for. It should have a rubber plug in it, but that hole is there for you to reach into the brake drum, pull pack the adjuster arm away from the star wheel, and simultaneously rotate the star wheel to back off the shoes away from the drum, so that you can remove the drum to service the brakes! As the drum wears, it will get a lip on the inside of the drum that will prevent you from sliding the drum off, unless you collapse the shoes inward so that the lip will clear the shoes. Make sense?
They make a set of tools for working on drum brakes, that looks like 1) a hook, to pull the adjuster arm away from the star wheel, and 2) a funny looking curved tool with a flat blade on the end of it. That is used to rotate the star wheel. Most people like myself just use something with a hook on it and then a screwdriver to do it, but you have to visualize the mechanism inside the drum that you cannot see, and operate it while blind. Some brakes have the access slot on the drum and other have it in the backing plate. In the case where the slot is in the backing plate, you now don't hook the adjuster arm and pull it towards you, you have to just push it away from the star wheel.
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