Minecraft – How to get out of the void

Well, first of all dark room mob spawners aren't very effective unless put to a large scale. I would suggest that you make a 30 by 30 platform which can have another platform 10 blocks above, it has to be nighttime and no other mobs are allowed to spawn around the building, also, you have to have holes in the floor with trapdoors, you can make the trapdoors activate at the hit of a lever. The reason why you want to use trapdoors that are open is because mob AI detect trapdoors as standing blocks whether they are open or closed, also, no water because of update with drowned. Also, its a good idea not to build on slime chunks, can decrease entity spawning. Lastly, one thing for you to check, do /kill @e to reset the entity spawning in area.

When you fly down, you sneak, like pressing the sneak button on PC. This means that if you press the down button and tap the object quickly, it will place it as if you were sneaking.