There’s not really a strong player in the open source search space.

Mwmnl exists, but per their own readme:

The quality is a long way’s off from matching the commercial engines at the moment

Kagi provides the source code for many of their services, but is not truly open source, especially because at its core, it relies on applying its own rankings to other’s indexes.

That said, search is a space where the old adage rings true:

If you’re not paying for it; you are the product!

For what it’s worth, I find value in my Kagi subscription.