Optimal distance to AFK for creeper farm?

I'm not sure they spawn underground as much as they do above ground. The game probably sees the farm as a house and there's probably code in the game preventing creepers from spawning inside player-built buildings, and probably buildings in general. It's been a while since I've played the game though, so correct me if I'm wrong.
-So maybe try removing the roof.
-It could also be the cat. Maybe creepers don't spawn around them out of fear.

It needs to be about 235 blocks on the y axis or the game will stop spawning mobs because on the amount of entities around the area so the higher the better

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.