Your rear beltwheel is worn out!
(As it is one piece with the hub we could also say the rear wheel is worn out.)
One can see that clearly on pics 4 and 5:
The higher area of the teeth (outmost diameter) should be shaped flat, just like the teeth of the frontwheel; yours are extremely rounded.
Below a new aluminium beltwheel and an older one with beginning signs of wear:
The effect of this kind of wear is also a reduction of the outer diameter and again this is leading to a loose belt. With a rear wheel like that you will never be able to adjust tension properly 