Is it going to remain tiny, with a limited number of maintainers? Are those maintainers going to stay put? Is it a hobby or a business?

I would pick something popular and unsexy; make it easy to google for advice, make it easy to hire folks, make it easy to onboard them.

A cautionary tale: my employer leaned hard into Elixir; but now, most of the people who know Elixir well have left. I am probably in the top ten, maybe top five, of the remaining engineers when it comes to Elixir knowledge, and I don't know much.