Not entirely sure what you're getting at but $0.02 worth of opinion says stick with the plan and go for the
Type IX. Few commanders were captains of all three main types, an exception being Fritz-Julius Lemp who commanded a
IIA in peacetime but already had the
Type VIIA U-30 when the war broke out and later got the
Type IXB U-110. A review of careers over at
uboat.net indicates that historically commanding (as opposed to serving on) all three types was quite rare. You probably don't want to get your
IX too late in the war so you can exploit all those remote objectives like America's east coast in early '42.
Good Hunting.