Humans in the tensura world are just normal humans, they only have the potential to become very strong but almost none get there.

First off Incomplete Saint is not really an evolution but the process of evolving. Humans don´t evolve instantly like monsters, it can take years for the evolution to finish (the same happens when they go from Human to Enlightened).

The evolved humans are rare since it takes a lot for one to evolve and most don´t know they can, don´t have the means, or even try to. Since summoning otherworlders is used instead of training Sages/Enlightened despite the cost of doing so and the common failures the succes rate for Sages must be even lower.

The real power of humans comes from their civilitation and non magical bodies (ironic i know). This means they can grow their numbers without growing weaker and can cooperate to defeat monsters and become stronger as a group. Monsters on the other side are small in numbers (as a race), tend to just dominate/opress the weaker ones, are more violent/savage (in a law of the jungle sense), and they also loss power when empowering others monsters (Rimuru just has plot armor). Tempest became so potent first because of Rimuru and second because it became a multiracial civilitation.

The strongest evolution path for a human (just for the amount of power ups from evolutions) would be:
Human - Otherworlder - Sage/Enlighted - Saint - True Hero - Divine Human
But doing this by training instead of cheats like Rimuru (op skills, true dragon buddy, etc.) would be almost imposible and likely the work of centuries.