Setting the 3d view function for a polygon layer

I am trying to create a 3d model of an archaeological site on QGIS. I had some contourlines, I made them polygons using the "polygonize" plugin and I categorized the polygons with a colour scale according to the altimetry value of each polygon.

At this point I used the 3d view in the layer properties panel to get a 3d view of the "polygons overlapping" effect. The value of the extrusion is the one found in the ELEV_2 field (i.e. the value of the height of each polygon).

The result is this:

There are two questions:

1. How do I keep the colour scale I used for the 2d symbology?
2. Is there a way to decrease the height of the base of the model?

I have already used Qgis2threejis. Unfortunately every time I try to open this layer with this plugin the pc freezes.

I have already tried this solution but applied to my case it does not respect the height increase of the polygons Jochen Schwarze’s solution