Build snake and lizard habitat. Frogs, toads and salamanders eat them too. Piles of rocks, wood, leaves or any combination of natural materials along with a water feature, will just about guarantee the arrival of some beneficial creatures that eat slugs.
There are very small bantam chickens that are almost exclusively insectivorous. They will eat the slugs and leave crops alone.
I see you've tried a number of measures. If I hadn't read it, few of those things would have crossed my mind, since I don't ever plan to put much labor or resources into slugs. They are the number one garden pest on this wet island and I've beaten them soundly using hungry critters.
Check out these two threads. ---
https://permies.com/t/38853/hugelkultur/Lizards-hugelkultur-home
---https://permies.com/t/37884/predators/Building-Snake-Lizard-Habitat-Winter