One thing is for sure, the “Germany cannot be ruled by a weak leader” part is a recipe for power struggles. Only the most ruthless, machiavellian sociopaths will rule.The author hasn’t revealed it yet assuming he did actually choose someone. Hitler’s favorites IOTL when the war ended in defeat in 1945 would be different than a scenario where the Reich wins the war and Hitler dies in the 1950s. Goering was his official heir until the very end but since in this timeline he died you can speculate endlessly as to who he would choose to be his successor.
IOTL Goebbels was his most loyal follower, Himmler was his second most loyal follower (“loyal Heinrich”) and the one carrying out the Holocaust, Generalplan Ost and the New Order and he became rather close with Speer due to his position and their shared interests in architecture. IOTL after Himmler and Goering were shown to be “disloyal” Hitler chose Admiral Doenitz as the new leader and also considered General Schorner and Model for the role.