Personally I wouldn't use the auto renewal as you are finding there is no renewal discount because the payment mandate isn't flexible.
I have been using avast free for a very long time over 13 years, so I can't speak from personal experience. However, what you have experienced is common, auto renewal happens followed closely by the offer before your existing subscription ends and before the new expiry date is applied.
There are some that go so far as to uninstall their paid version as that offers to install avast free, once you have avast free, you often get offers.
Some may even seek a refund, Refund -
https://support.avast.com/support/tickets/new?form=2 - all of the payment or the difference between the two payment and offer.