Optimal distance to AFK for creeper farm?

I build this creeper farm with a cat and waterflow mechanics to farm gunpowder but I cant find a good spot to afk in.

here is a screenshot of the place, I've put carpets to prevent spiders from spawning and I've put wooden trapdoors on top so only creepers can spawn in here.

The cat in the middle scares the creepers that then fall in the water stream and get taken to a magma block which kills them in about 20 seconds.

It's built under the ocean and I've lit up the caves surrounding the farm so spawn rate should be good.

When I come close to the area and enter the farm I get a few creepers spawning and then it all shuts down.

I've tried standing here -

which is about 30-ish blocks away from the center of the farm

and I've tried standing on top of it like this

I've been afk for hours but no creepers spawn.

I know that mobs will not spawn more than 128 blocks away from a player, and if they are 32-128 blocks away from a player they will start to randomly spawn and despawn. The optimal distance I thought would be around 30-ish but the creepers aren't spawning.

Does anyone have any idea why they don't spawn at all and at what distance I should be AFK in order to get it to run?

It is good that your farm is low in the world if you are lower there are more spawns, however you need to light up all the caves within a 128 radius. Otherwise mobs spawn there filling up the mob cap in a singleplayer world this is 70 mobs, if the mobcap is full, there will be no spawns.

The entire area where mobs are meant to be (spawn platforms, drop chute, kill platforms) must be within 128 blocks from you - any farther and the creepers will despawn. Also, at least the spawn platforms must be more than 24 blocks away; mobs don't spawn any closer to the player - so when you come to peek into your farm, all spawns freeze. Anywhere in between is fine.

So, for optimal results, your afking platform should be slightly less than 128 blocks above the lowest part of the farm (the killing floor); additionally to benefit from the low location of the farm in increased spawn rates, it's good to have anything above the spawn platforms be as low as possible - so build the platform slightly to the side, not directly above the spawn platforms.

Note lighting all the caves within 128 blocks (sphere) from your afking spot is crucial. Even a small unlit area will keep spawning mobs eating into the mob cap and gradually making your farm grind to a halt as a crowd of mobs accumulates in a single unlit cave. If you're unable to find all the spots, a rather poor but better than nothing way around that is to despawn all mobs periodically - so you may expand your elevator/tower of your farm to (62+128=) y=190, a spot you'd visit occasionally to despawn all the mobs in missed, unlit caverns (but also unfortunately inside the farm too) and restore the rates to 100% for a time.

Let me add, utilizing the ocean bottom was a pretty smart move as only a little section of the sphere remains to be lit up. You will get the drowned occasionally spawning and spawn-proofing against them is nearly impossible, but their number is sufficiently low they won't ruin the farm.