Hi! I have a dog that can't do Trifexis either, she just throws up. There are a couple options, you can do a heartworm preventative plus a topical flea treatment like Frontline or Advantage. My vet has my girl on a heartworm pill plus the new Seresto collar, which repels ticks and treats fleas. I like it a lot, it isn't like the old collars, which are terrible for your pet. The prescription collar has the same kind of medicine in the topical flea treatments, but it is released slowly through the collar. It is good for eight months or five months if your dog gets wet a lot.
You may ask your vet for their opinion, since they are aware of the pests in the area and the best way to treat them. Hope you find something that works!