12mm looks pretty good - less metal removal than 1/2-20 but can withstand more torque (+16.6% more X-area).
12mm × 1.25mm pitch is a common size, and by happy coincidence works out to 20.3 tpi, so very little material will be trashed from the existing threads - they'll just be deeper.
Remember that if you use fine nuts reduce the torque, since it's inverse to the pitch.
