You can handle your meshes procedurally in unity. For this take into account that it quite a low level an time demanding stuff (although it is so awesome).
For better advice if you are trying to do this yourself, you would need to share your attept and your code for more specific help. I am not familiar with probuilder, but you should be able to access to the vertices of the mesh you create.
Meanwhile you can consider external tools to reduce your vertices or polygonst such as this asset to be used to solve your problem, or to check it souce code to be used as inspitarion.
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