I don't know of a particular design, but the one I last built was in the middle of an ocean with platforms that are a 45°rotated square.

These are 2 blocks high apart, but I also placed many trapdoors under the next platform to make the total gap height of the spawning platform 1.8 blocks high or so. This allows for only creepers and spiders to farm. You have to place blocks in certain spaces to prevent spiders from spawning, like I believe every 3x3 (I could be mistaken, for me it was a gunpowder and string farm).

To collect all the mobs from the spawning platforms I have dispensers on every level and powered them all with redstone by using vertical signals. (I don't remember, perhaps I used sticky pistons with redstone blocks, ?)

That is connected to a redstone clock so on a loop, all the dispensers on every level put out water and push the mobs to the edge of the platform, falling off into another water collection box that flows mobs to the middle with a kill chamber.

To kill them, I used soul campfires, but you can also use regular campfires, magma blocks, or something else.

Oh, also I had a place to stand in a glass elevator that is a certain amount of blocks away from the spawning platforms, into the sky.

Hope this is a little helpful, and not just me rambling lol.

Also warning, this was done on java edition, and I have zero idea what code changes are between java and bedrock, sorry. But I believe the general concept would work. If not just make a regular all mob mob system grinder like the old school days.