I'm afraid I've been there, done it, got the T-shirt, DVD, mug, and bra-and-knickers-set to match with expensive supplements over the years!!!
My trad-lad has sweet itch, and he has: Brewers Yeast, Micronised Linseed, teaspoon of Cider Vinegar and when the sweet itch season is active I give him Clivers (sticky stuff, grows in hedges).
Because he's got sweet itch, I avoid garlic.
The most effective thing you can do to guard against flies is to cover the horse up, first and foremost, and/or make the coat unattractive to flies, either from inside out, or outside in, if that makes sense.
My welsh has sweet itch and i find the D-itch makes a big difference last year was the first year he kept his full mane and had the odd day without his SI rug on