I'm in N.Carolina where the mosquitoes can come out just about any month. Just a couple of weeks ago it was in the 60s. So down here I have to give the heartworm medication year round. In your case giving it in the months it's warm enough for mosquitoes is the correct thing to do.
Expressing the anal glands is something I never caught onto. When you take the dog into the Vet to have them expressed, ask them to take a fecal sample to look for worms/eggs. You definitely do not need 2 trips.
The more tissue or weight in the rear end, the less force the dog has to express the glands. So weight can contribute to them plugging up. Diet of what goes in has little effect. It's how it comes out that's important, where a soft bowel movement does not create enough pressure on the anal glands to express them.
Basically large muscle/fat dogs tend to have the problem more than other dogs. My previous dog had a lot of anal gland problems. The current two- none.