Even iterative solver cannot solve large number of elements?

Can’t you simplify the model a bit ? Are all those small features absolutely necessary ? It’s better to avoid such huge meshes.

Try with Gmsh. Many models can be partitioned in such a way that they become sweep-mesheable. Otherwise, you have to stick with tetrahedrons since the hexa block technique is usually not available.

Give it a try: Calculix with pardiso OUT-OF-CORE mode

But most likely you will end up having to reduce the mesh density anyway.