I've done the run several times in ships. I also tuna fished about 1200 miles east of Japan
. Your time period is the end of winter. Watch out for storms. Typhoons can build some huge waves SE of Japan. Even when they're 1000 miles away. I've been in 80'+ swells but far apart. Never seen the waters S & SE of Japan smooth like the tropics. Near Guam
and probably for 1000 miles around, the water
is ok as long as there's no storms.
Following the island chains down, you have several islands you can get behind for protection, but not a decent harbor until Saipan. If you cruise
any of the small volcanic islands, be careful. Many of their surveys are from Japanese charts
and were made before WWII.