Well, we have https://mindustrygame.github.io/ that it is free software. And IHMO it is best that Factorio.
Factorio is about the long game, and your designs have to be able to scale, especially if you're going for mege-base scale. It's more about complex designs and a very deep tech tree and dependency hierarchy. It's about factory automation at its core, with some PvE/tower defense (optionally) thrown in.
Mindustry is mostly a tower defense game at its core that uses automation/factory building to accomplish that goal. It simplifies a lot of things that are more of a challenge in Factorio. E.g. not needing inserters makes optimizations much easier. Also, the way building happens almost immediately and you don't need bots makes construction much easier.
I like both quite a bit, but depending on what you're looking for, either game could be more enjoyable for you.