As I approached 30 I felt what I now understand as anxiety. Nothing crazy but you wake up one day, you're a year old, you take stock of your life and see what you've accomplished. I chased credentials and new jobs, did well but not yet able to retire. Old relationships grew strained and new ones are hard to form. Etc. I was definitely more afraid of death than you are.
I stumbled into children. Met my wife, didn't overthink things and decided to marry her after several years of courtship. She wanted children so I went along with it.
After they were born, the anxiety went away entirely. I just watch them grow older. It might change after their grown but hopefully they'll have children and I'll be able to repeat the process (my parents sure have).
To answer your question, I don't see a way I could have been fulfilled without children. It brings a lot with it as well. Before children the worst thing that could happen to me is dying. After kids you realize there's a whole world of potential pain and sorrow that is now before you. You're also constantly reminded that every kid is a roll of the dice when you see others in a similar spot as you have children with health or behavioral issues. It's terrifying.
But for me life shouldn't be all pleasure. I've always liked exercise because it does feel terrible while you're doing it. But at least you're doing something. I feel that way about children.