How to keep certain faces planar while decimating mesh For a papercrafting application I need to decimate (collapse) the geometry in order to reduce the number of faces. The original geometry looks like this:

When applying the decimate modifier (with triangulation), I noticed that the flat base of the spherical dome is not planar anymore:

I want keep this face planar (which is intentionally so in order to be glued to something else) while collapsing.
Is there a way to constrain the vertices of that face such that the decimate modifier will be able to move them only within the plane of that face?
Since I'm still a newby to Blender, I wouldn't know where to look for a solution. My searches for obvious keywords did not surface anything that looked applicable to my problem.
Use checker deselect to dissolve every nth top and bottom vertex or try the Decimate modifier options (co-planar etc.)