I'm way out of my league here, but why not make the floor out of a block that mobs can't spawn on? Pretty sure half-slabs qualify, but also stairs (no good for obvious reason) and doesn't regular glass prevent spawns too?
I once watched a youtube of someone making a Witch mob grinder... and the first thing they had to do was clear the entire area around the witch hut for a radius of 128 blocks or something ridiculous, and put down a half-slab on every block, so no other mobs would spawn in the area. Fun, fun fun!
Something I've been wondering... since half-slabs cause issues with machines looking like they're not sitting on the floor, can you still put two half slabs down to prevent mob spawns but it looks like a regular block?