Was just looking into this (again), as we started to research it a couple of years ago. Regarding prorated fees, this is from their FAQs:

For existing members as of May 1, 2019, the ABA annual membership year is from September 1 through August 31. For those joining or re-joining in the future, your membership year will start on the date you join. Dues are charged for the full 365-day period. We no longer prorate dues for partial years/periods.