I took a second look at the screenshot of the sub techs. Here are my concerns:
The early Uboots did not really have sonar. Sonar is an active detection mechanism for stuff that is underwater. Uboots where silently hunting ships that are at the surface. So not only would they not able to find anything but enemy subs (and sub-sub fights were extremely rare in WW2), it would also give their position away. Only later in the war did the Germans develop advanced echo chambers and passive sonar detection. I would move the historical year for this technology back a bit.
Note that the British did have sonar at the beginning of the war, but as said, it was mainly an ASW weapon.
Hydrophones just listen to fast screws. If the 1940 one is the second iteration, that seems fine.
The Germans developed radar detectors (the Biscay cross for example) for their subs before they had radar on them. German radar in general was lacking with the British and American one, and even then, miniaturizing it to fit on a sub is a big problem. Donitz for a long time did not believe the Allies had radar small enough to fit on air planes. The first radar detectors showed up around 1941/1942. Radar should come after that. I would swap the historical years for detection and radar itself.
The uboot resupply at see, the Milchkuhe (milk cows), came into action in late 1941, early 1942, and made sure that type VII could participate in Operation Paukenschlag, the raiding of the US East Coast. Before this, the Type VII could run on one engine, and then make it, hunt for about 10 days, and then return home again to France. With the resupply, they could stay about 2-3 weeks longer. So I would make the historical year for this technology a bit later, 1941 or 1942.
As for the XXI, key innovations where more efficient engines, a creep motor for the silent run, as well as the streamlined hull, designed to run completely submerged. The radar and snorkel could all be retracted into the hull, and the AA gun was sheltered. The XXI did sacrifice the deck gun for this, though. The ability to run underwater for so long was due to the vastly increased battery capacity. Type XXI did not use air-independent hydrogen peroxide engines that was in development at the same time, a technology later made absolute altogether due to nuclear subs.
Good luck with the DLC.