What Peter suggests is simple and great suggestion. Also, please don't expect uniform load transfer across threaded joints. This phenomenon is not only for threaded joints but almost all structural joints that are not optimized to shear in the loading, so as bonded joints, brazed joints, etc.
One other option is to use simplified thread modeling which is detailed here. This can capture the stiffness response, but won't get you the thread root stress, which the mesh in your model can't get anyway.
37.2. Problem Description (ansys.com)
Finally, you can convert a cylindrical joint into a screw joint with a few APDL commands in a command objecy, so you get the twist extension relationship without modeling any threads.
Best of luck.