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?

How is your current level? Given you've had N1 already, I assume your Japanese proficiency would be quite good? My basic idea is to change the role to which requires Japanese at a bit higher level than current your level

I personally recommend you read mangas about work related troubles (lol). Manga helps you a lot to learn about not just terms that are frequently used in Japanese business (such as 稟議, 掛け合う, 同席 etc), but also about Japanese work culture in general. My favorite is 社内探偵 and 私以外みんなバカ by たかおかみさお. The characters in たかおかみさお mangas work in IT ventures or startups so it’s good for you to learn about the vibes in these kinds of companies. 島耕作 is an old one and I heard it’s good but quite old and the manga is more about old traditional Japanese companies I guess.

I am a native Japanese speaker, but I myself learned a lot of words from たかおかみさお before working as a fresh grad (新卒社員) lol.