One way to think about this subject is in terms of bargaining power. An unconditional German surrender eliminates all German bargaining power. That would leave the Soviet Union and the Western democracies with all the bargaining power, and also in a position to collectively determine what to do with postwar Germany.
It’s also worth considering the level of negotiating determination, and negotiating objectives in the first place. Churchill was interested in a democratic postwar Europe. FDR’s interest in spreading democracy or resisting communism was much weaker than Churchill’s. On the other hand, a Soviet-American postwar alliance was very important to FDR. There is little reason to suppose that FDR would raise strong objections to the Soviets getting about half of Germany’s prewar land territory. (Which was about what they received under Yalta.)
If things went badly for Stalin at the negotiating table, he could always point out that the Red Army had the ability to take what it wanted by force. Between the power of that threat and FDR’s intrinsic lack of interest in stopping the spread of communism, I think it would be very unlikely that the Soviet Union would come away from an unconditional surrender with anything less than half of prewar Germany under its control.