It is a rainforest in the winter and a desert in the summer. Very acidic soil. We get plenty of slugs but it isn’t too big of a problem. I had way more of a problem at my last place. I wasn’t sure if iron would be problem. But like you said it’s probably locked up or if it isn’t its presence does not cause a problem. Biggest problem I have is them burrowing in the lettuce. If you have a severe problem try sample spraying see what it does. But if it rains a lot you will probably have to keep replying.
Discussion
That's interesting perhaps it's not in the right form for it to be absorbed too. Apparently ph and eh can affect it's availability and certain rhizobacteria. Potassium makes it more available too. Perhaps some oxidised hot compost which you added some wood ash too might be worth trying. Certainly would raise your ph.
Going try and foliar stuff next year, most things were not that bad here despite it being very wet....perhaps the em rhizobacteria helped but the hostas got hit badly by them so that would be a good test for the iron feed.