Please take a look at the image below (click it for full size):
According to mob spawn rules, mobs can't spawn within 24 blocks of a player, which is currently okay, because the first spawn floor is 24+2 blocks above the AFK platform.
However, wiki says that mobs further than 32 blocks from the player don't move, which means they will never fall through the trap doors and so on.
The third biggest issue - The grinder stops working as soon as there are 63 mobs at the bottom of the kill tunnel. This can't be a mob cap, because the grinder is over 130 blocks above ground. What could be the reason?
I would appreciate it if you have any other suggestions. I used this tutorial:
And this image as reference: