Your vet is absolutely right for wanting to do a fecal on your dogs. Canine hookworm resistance is a massive emerging issue and so it’s important to make sure that the Simparica Trio is actually working. If it is, then there’s no reason to give them another drug. It’s not a cash grab.
You’ll want to avoid being barefoot, cover any cuts/sores, and don’t roll around in that grass for awhile. You can, in fact, get canine hookworm. It’s fairly common in dog parks as well, so that’s fun.