How do you niche down would be my question here.

I am 40 next year and have missed the opportunity to niche down - I work as Tech Product Manager and enjoy my job but find very little opportunity to really focus on something I truly enjoyed. Either the startup run out of money, the project in the large org got cancelled or there was simply not product role available.

I'd be glad to know if somebody has an advice on how to niche down (technology, domain, personality, branding etc.) Thank you