Why does it looks so much darker in cycles? I want it to look more like in Material Preview/Eevee. I'm not experienced with shaders. I didn't made this material myself. So, I'm sorry if this is a stupid question.
Shader to rgb node is for Eevee only.
The documentation mentions:
Using this conversion breaks the PBR pipeline and thus makes the result unpredictable when used in combination with effects such as ambient occlusion, contact shadows, soft shadows and screen space refraction.
Edit: though, I'm not able to reproduce your issue...