The reason that Point3d does not have dispose, is that it is a value-type, meaning that it is allocated on the stack, rather than the heap. When these go out of scope, the memory will be cleared. Any Point3d in a collection (list, dictionary, …) will need be garbage collected when the collection is collected. You can force garbage collection using System.GC.Collect()
The way I would approach this is with an RTree data structure of the point cloud. Unfortunately, the RTree in RhinoCommon is too limited to support searching on different levels in the tree, but there are python packages that have RTree functionality so you could use these I guess.
The idea is that an RTree contains hierarchical data on bounding boxes that contain points. So, you would traverse the tree from the root on up and test if the bounding box at each level is contained within the Brep. If at one level it is contained within the Brep, you don’t need to test the higher levels of the tree. This way, you can more quickly decide if a (possibly large) set of points is inside the Brep. The overhead of creating the RTree will likely outweigh the is-inside-brep test of 20M points.