Is it cheaper to pay monthly or annually? Also, will my car insurance premiums get cheaper if I stay with one insurer itself?

It is always cheaper and cost-effective to pay upfront annually. When it comes to premiums getting cheaper if you hang on to a single insurer, this is unlikely, particularly in the long run. If a customer does not look around for better and most effective deals, the insurer can take advantage of the situation and may charge you higher renewal premiums.