William got elected as king of the Romans somewhere in 1520 ish I believe as Frederick's successor as he has no heirs of his own. Did Frederick have a coronation or not in this story? I haven't written one, mostly because I intend to keep the Empire on the fringes in this story, but he might have had one regardless. The Bavarians pushed for their own election as relatives of the Hapsburgs, but they compromised with Frederick to be elected as imperial sucessors instead, knowing that they only had to wait for a while. So that is what happened.
Is it hyper realistic? Perhaps not, but I intend to keep focus on England, Spain and France for the most parts, so I don't want to get bogged down in imperial scheming too much. The Bavarians will be emperors from 1525 and Louis and Mary got Austria, Hungary and Bohemia. So there is that.