Well, firstly, you dug a gigantic hole and are only using the bottom of the "bucket". Add more platforms to increase the amount of spawning space and you should see significant increase in spawning.
Secondly, your glass ceiling is too low. When adding more platforms, you'll want to take full advantage of the entire slime-spawning area (that goes all the way up to y=39). I put my finished ceiling (anything above that would be technical stuff dealing with redstone or other structure I don't want to really look at) at y=43, which guarantees 3 full blocks of space above a spawning platform that lands on y=39. I never remember where the final platform lands, and since the bedrock layers are uneven this can vary a bit.
Thirdly, where's the sky? You dug a gigantic hole for the slime farm, don't ruin things by not finishing the job. With your glass ceiling being at y=38, by digging out the hole all the way to the surface you've shrunk the world height there by a factor of about 4 or 5 (the actual shrinkage is more than that, but it consists of air that the game ignores completely to begin with). Being in an extreme hills biome that has land at levels higher than sea level, this basically means the spawning is spending more time trying and failing to spawn mobs outside of your farm than inside it. Remove the land above your farm and this issue goes away completely. Note that this does not mean you cannot build structures in this space versus leaving a gigantic open hole to potentially fall into, but any building in the hole above the slime farm needs to be made out of leaves, glass, and other blocks considered transparent.
If you don't mind the cheating, take a dip into the world using spectator mode (/gamemode 3). This is the official implementation of x-ray mode by Mojang, so whenever your head lands inside a solid block you can see all the stuff that exists underground where you normally couldn't see it. It will give you a very clear picture of just how thorough your cave lighting and spawnproofing work has been (hint: most of the time it's not as thorough as you expected). Flying around in this mode can also give you great appreciation of just how large the mob-spawning zone around the player actually is.