I've been using coconut oil on my dog's fur for quite a while now. When we go outside, I don't see any flies or mosquitoes going on her, but maybe it's not warm enough where I am. I however, will encounter a fly, but her, none. No fleas on her since using it either and we have cats who have them (still trying to find a product that works on them, they are not adults yet and what we have only works so much).
Vita Coco Coconut oil (it is a solid at room temperature) is what I have (Walmart, under 8 dollars). It's natural, cold pressed, virgin and not processed and non-toxic to dogs. IDK if yours will be sensitive to coconut smell, as you have stated that Tanner is sensitive to peanut butter.
Be careful with essential oils, though, as some are not good for dogs, and some natural remedies have contraindications with seizure medicines. You may want to research interactions with herbal/natural remedies with medicines beforehand. Like with phenobarbital-> bitter almond, sage, cedar leaf and cedarwood oil, elecampane, eucalyptus, evening primrose oil, gotu kala, ginko biloba, hops, kava kava, lemon balm, english lavender, german chamomile, st. john's whort, valerian, wild carrot, wild lettuce, yarrow are all contraindicated. (There is more, but just to give you an idea).(The Essential Herb-Drug-Vitamin Guide). Some natural remedies for fleas, ticks and mosquitoes have lemon balm and cedarwood or cedar leaf concoctions.
Happy to hear Tanner is doing better.
Hope you can find something that works for Tanner.