Improving without mentor [closed]

Question

Although I consider myself not much experienced, I keep landing in jobs as the leader somehow (in the current one I am tech director of the company, in a previous one I was the R&D head).

I understand why this happen (I have a certain specific set of skills, and I use some uncommon programming languages), my issue is: I cannot find a mentor...

I know there must be stuff to me learn, and I notice as I work over the years that I am slowly learning more and more, but I believe that there are some way to get better much faster than just coding...

So, how I improve when there are no other person in the company to teach me, or to me read their code? (or fix their code even...)

I think I am beyond books, but way below a "master" level, so I don't know where to learn more.