If you have N1, that already is impressive and me being a native Japanese speaker, I honestly don’t think I can pass that exam.
So im guessing your baseline is good in regards to the vocabulary, reading and listening in general.
Maybe look for a job where Japanese is required and work in that role/environment for like 2 or so years?
To learn real business level Japanese, sticking yourself in the environment seems like the easiest option.
But at the same time, not trying to be rude or anything, as long as we can tell you are a non native, I think Japanese people these days are a bit more lenient towards the non business level Japanese so another option will be to start the business as a side hustle in the meantime while you get your business Japanese up to par?