If you're going to do in-depth work on the IJN you simply will be spending money and--if possible--learning Japanese.
I might point out that increasing access to the language is growing but a great deal of material that I've run into is handwritten and, of course, pre-1945 when the language underwent changes both conditions of which provide for any number of landmines, especially the handwriting.
However, with effort, one can ferret out a great deal of information which is not available in English, particularly reminiscences. One example of ferreting I can point to are translations I did for destroyers Suzunami, Hamanami and Naganami. Turns out there is more to the stories of their losses than what we currently see in English.
And I have a ton of information on another -nami but that's going into something rather special.
Trivia, I'll readily admit, but one more piece of a puzzle.