This is a bookmark-worthy post from the venerable ER pioneer John P. Greaney on negotiating the Part D quagmire. IMHO what makes it worth referring to repeatedly is the extensive series of links he provides to other resources - along with the reminder that all of this stuff really needs to be revisited annually.
https://retireearlyhomepage.com/medicare_partD_2021.html
https://retireearlyhomepage.com/medicare_partD_2021.html