Well, maybe the question is not about how dumb Doenitz and his asses were.
Wasn`t it a guy named Churchill that feared Doenitz and his subs the most?

So, there shouldn`t be any discussion how dumb they are and even a viewer of the movie "Das Boot" could know that the Captain complained about 2 things:

* being directly attacked out of nowhere in darkest nights ("Zerstörer aus Lage null")
* empty sea and no convoys that feels like the convoys were sailing or led around the subs and its wolfpacks

The commanders complained to the flotilla commander and they reported to Dönitz. Also Dönitz went to the Flotilla harbors quite often visiting crews and commanders or inviting them to understand what was going on board from first hand - not filtered by anyone else. Pretty unusual for that days.

So Doenitz was asked a lot about these 2 topics and therefore asked the experts and military intelligence regarding RADAR and ENIGMA decoding possibilities ... cause since the first subs survived direct nightly air attacks in the gulf of biscaya and reported the former complains of direct attacks by destroyers in terrible dark, stormy nights "aus Lage 0" had become no longer a question of "incidantly attacks".
At that point the Doenitz staff had asked over and over again if it was possible to create a radar fitting into a desgtroyer and later plane. And then research began to develope even radar absorption snorkels.
But ENIGMA has been a secret since mid of the 70ies which allowed to avoid that the far bigger amount of german subs operating / searching in wolfpacks were encountering more convoys. If these boats had been searching alone the likelihood to find convoys would be even higher for them. But acting as wolfpacks or "Vorpostenstreifen" would make it a lot easier for the british to redirect the convoys around the known german sub positions.

And the german communication intelligence answered always that the decryption of enigma messages was impossible. What should Doenitz do else?
They added additional security improvements over the time like another cylinder. And not to forget: they did not know about the raids against german subs to get the enigma keys like that of U 505 and others

To question how thumb Doenitz and his stuff must have been is simply ignoring the facts cause why should all the american navy, airforce, tank units have searched for german experts to gain or benefit from their knowledge and expertise like that of sub type XXI construction, the tank construction, the 8.8 anti tank gun, the V2 techniques and the ME-262 jet?
They simply wanna make sure not to oversee anything and to benefit the most from that expertise, also of the submarine warfare under Doenitz.

Today you can nearly daily read on twitter how dumb american experts and advisors are seen and treated by someone who sees himself as the biggest expert for everything and who does not need any advice beyond google and twitter.
Well, we had such of these guys at least 2 times in the last 2 centuries and the simple german worker and his family from the street had to pay the price for these guys. And therefore germans are pretty cautious regarding such type of nice weather leaders or captains who believe to fight efficient against a flood or storm from the office / desktop - not from the battleground of the surving fighting for their lives after a flood or storm.