I wanted to post of pictures of the grinders on SDF-MC.
Creeper Grinder (this one may need some work, not generating as many as expected) *removed externally hosted image*
Guardian Grinder (This is pretty brutal. Plenty of food there but have diamond or netherite armor with mending) *removed externally hosted image*
Loot Chests at the Guardian Grinder *removed externally hosted image*
Slime Grinder *removed externally hosted image*
Warp Room (This is a warp room at the Guardian Grinder that has warp buttons to all grinders (Save on warp slots) *removed externally hosted image*
Triple Cave Spider Grindere *removed externally hosted image*
Magma Cube Grinder *removed externally hosted image*
Enderman Grinder *removed externally hosted image*
Wither Rose Grinder *removed externally hosted image*
Ghast Grinder *removed externally hosted image*
It's a bit of that, yes. There are some height and block adjustments you can make so only creepers can spawn in it. Also, they are afraid of cats, so they'll walk off the edge to get away from them. I THINK something has changed in MC though, because this one stopped working. I tried a couple other variations, and it has not improved. I have to dig into it a bit and get it working again. I just revamped the Wither Rose and it's working crazy right now.
Ah I see, that's unfortunate. Hopefully you'll be able to get it working in the future, sometimes changes can be a minor setback, others they can be devastating. I remember my friend made a slime farm in a Swamp (this was a long time ago) then they changed it so other mobs spawn in Swamps and Slimes only spawn in Swamps on the full moon, that change essentially rendered his farm useless. Hopefully this change won't be as major a setback.