No Mobs Spawning in Mob Grinder

I think the issue you're running into is the size of your platform is very small, while you're built very high up in the air. How spawning works in Minecraft, simplified:

1. It picks an X/Z location to spawn in
2. It picks a random Y location from 0 to the highest opaque block above it. So if the highest block is at y=15, it picks from 0 to 15. If the highest block is at 255, it picks from 0 to 255.
3. Look at that X/Y/Z location to see if its a valid spawn location (has a flat surface underneath, is sufficiently dark, the player is between 22 & 128 blocks, etc.)
4. If it is, it then spawns 1 mob at that location, and 2-3 of the same mob nearby (using a similar, but simplified algorithm)

What this means practically is that the higher you build your mob spawner, the more spawning spaces you need to get good rates. 400 spawning spaces at y 200 is going to be too low to get many spawns, especially if you're checking it fairly regularly to see if anything spawns.

Other potential issues:

1. The farm you're using relies on mobs walking into the water themselves. They'll do this if you're standing between 22 & 32 blocks from them, which it sounds like you're doing, but it's fairly slow, which will contribute to you feeling like you're not getting drops.
2. The farm design you're working with is very old (from 2015), and mechanics have changed a lot since then. In particular, water pushes mobs more slowly the longer the stream, and if it's pushing them at the 8th block of flow, it won't push over the edge.
3. While mobs will instantly despawn if they are ever >128 blocks from a player, they will also be chosen at random to despawn if they're greater than 32 blocks (this number may not be accurate, it might be higher) from the player. The chance of a mob being chosen to despawn increases the farther they are from the player. So, the mobs you're dropping may be despawning as they fall down the tube.

But, the main issue is the number of spawn spaces.

For a more in depth explanation of the mechanics, and a more up to date farm design (though it does still suffer from issue 2 described above), see this video: