This is not an evolution-focused novel. After the initial arc where the story plummets, there's no further development of evolution. The protagonist gains levels by killing as a human and consuming as a monster, but this doesn't happen consistently throughout the 120 chapters. In fact, I recall only one instance where his level was mentioned, and he was surprised by its unexpectedly low value.
The few instances of "evolution" felt forced, with the author mentioning it every few chapters, but it didn't feel organic or substantial. For example, when the protagonist battles an S-rank adventurer, he nearly dies, but there's no mention of his evolution helping him or his growth afterward.
When he evolves from a fish to a snake, he abandons his isekai companion, who later reveals that it took three days for him to evolve into a form capable of traversing land. Around chapter 200, they face an S-level dungeon, and no character levels up despite the high stakes. It's clear that the author might retroactively grant levels later in the story to justify plot developments.
In summary, the evolution aspect of the story feels underdeveloped and lacks progression beyond a superficial level.