GMS (Group Medical Services) has an option that includes a modest amount for pre-existing conditions, which also covers vaccinations like shingles, not covered by all provinces.
Extended health insurance is to cover catastrophic new developments, like a bad car accident or major illness or an organ failure. It is not a buying club for you to save money on every day treatment for your health as it is today.
From my HR days, health insurance companies are aiming to pay out 91%. The 9% covers all their costs and their profits. If they pay more than that, they raise premiums.
I have extended insurance, with a premium prescription policy. I've opted out of dental, because it is much more predictable and and manageable and unlikely to result in savings.