I found out why my render is limited to 32 samples:
At some point I wanted to override the material and I left 32 samples in Material override samples field.
But I still have those artefacts (tiles) which my CPU or GPU left behind :(, and had some strange line in the middle of render...
This is probably happening because of very narrow triangles going back-and-forth across the surface.
I might try using Beautify Faces if you are willing to convert it to mesh, but if you don’t want to do that, try the Remesh modifier with various settings.
To improve the shading:
either check ✓ Harden Normals in the Bevel modifier.
or slightly inset the area
FunFact: I'm not quite sure why this helps, it'd be great if someone explained it in the comments :)