Slimes barely spawn

I'm having trouble with slimes spawning. There have been like 4 baby ones that spawned over the period of an hour, but thats it. I was going to make more layers but if there is something I need to do different I need to know. Also according to 2 sources I am in a slime chunk and I have lit all nearby caves so they don't spawn any more mobs. Is there anything I'm doing wrong/need to implement?

First off, you only have 1 spawning platform. The rates won't be great without adding more.

Second, the most probable cause is that you didn't light up some caves. It's very hard to do without spectator mode or a freecam feature of a modded client, and while unlit locations over 32 blocks from the player's afking spot will severely impact the rates, unlit locations within between 24 and 32 blocks will completely ruin them, the farm completely grinding to a halt after half an hour or so (random mobs spawn and occupy the mob cap within the 128 radius around the player, with exception of 24-block internal exclusion sphere. The mobs over 32 blocks from the player despawn randomly though, so the rates of the farm suffer but it eventually does spawn something. Mobs within 32 blocks don't despawn at all, so if they fill up the mob cap, the farm just stops.

And there's the last option: in your impatience you wanted to see the farm in action so you afked right at its edge watching the farm, and encompassing its spawn platform with your 24-block spawn-proof sphere, so the only slimes you'd see are the ones you find when arriving there.

Edit: in 1.14-1.15 (not sure about 1.16 but should be doable too), you can use an X-ray glitch to easily find most unlit spaces in the area. You need a composter, a piston and a lever, plus a pick or a shovel.

Dig a 3-high 1-wide nook, and place the composter at the end. Get in the composter, place the piston over your head and a lever next to it. press F3+B to display hitboxes of all entities (mobs tend to be hard to spot in dark caves). Activate the lever and you'll be squashed into the composter and be able to see through the walls. You'll need to break the piston or the composter to get out.