This is another similar question I answer this month. The previous question I answered was https://clarity.fm/questions/6150/new-coo-clashing-with-two-most-important-veteran-employees

I was requested for a call due to my answer in the above link. However, I asked him to follow my answer first and if he can solve the problem after using my answer then no need to request a call from me.

Luckily, he successfully used my method and save the money to call me. Thus I suggest you may click the link to read my answer, it may have some helps to you.

There is only one thing I like to highlight based on your case:
"you greatly depending on the employee really makes me wonder should you cease operation or if your profit still very attractive, then treat that employee as a customer and bear with his nonsense (although I feel quite ridiculous, if he really indispensable, you can't do anything much but just have the two options I suggested above)"

"if you disagree that he is indispensable, then my link above will be useful"