I think "Founder" is the correct job title, and use the description of the role ...

cjbprime on May 9, 2018 | parent | context | favorite | on: Ask HN: How to explain being sole employee in job ...

I think "Founder" is the correct job title, and use the description of the role next to the job title to explain that you were working solo and responsible for everything. You don't have to fit it all in the job title.