The last Japanese I-boat to undertake a Yanagi mission was sunk in the Atlantic in June 1944 on its way to Germany.
The last U-boat to leave for Japan, U-234, left Germany on 25th March and Norway on 15th April 1945. So too early to be of assistance to an escaping Hitler at the end of April. It surrendered off Portsmouth USA on 19 May 1945.
H-Gram 033, Attachment 1 Samuel J. Cox, Director NHHC July 2019
Many U-boats attempted to escape German and Danish Baltic ports in April/May 1945 but had to run a gauntlet of Allied minefields, Coastal Command Liberators and Strike Wings and, by the beginning of May, Typhoons of 2nd TAF (the last are credited with at least 3 sunk) as they negotiated the Kattegat and Skagerrak. Sinkings in the area went on until 5th May. But these U-boats were what was left in the training flotillas or were working up.
Scuttling of the incomplete and otherwise unready U-boats went on 1-7 May 1945.
Operational U-boats were by then based in Norway, with the last beginning their patrols in the first few days of May. One of the last to leave was Type VIIc U-977 on 2nd May. It finally surrendered off Argentina on 17 Aug 1945 which included a spell of 66 continuous days underwater.
The U-boat War in World War Two (Kriegsmarine, 1939-1945) and World War One (Kaiserliche Marine, 1914-1918) and the Allied efforts to counter the threat. Over 40.000 pages on the officers, the boats, technology and the Allied efforts to counter the U-boat threat.
Not sure how Hitler would have fared. The journey apparently drove a number of the crew to the edge of nervous breakdown.
The other U-boat to surrender off Argentina was U-530 on 10 July 1945. But it left Norway on 3 March 1945.
I doubt he would have headed for any of the former German colonies in Africa. Britain & France had them firmly under control. French Cameroon went Free French in 1940. The German regime in SW Africa / Namibia pre-WW1 had been pretty brutal on the natives, so I doubt his presence could have been kept secret for long.