If you mount it sideways, you should disable the automatic keystone correction. It works using an automatic level sensor, and when sideways might keep trying to adjust for a level that isn’t there.
Perhaps an app like Set Orientation can be used to force the PPM (or at least the apps you use on it) to change the orientation to match the side you turned it to.
Be aware though that the cooling and heat distribution might not have been calculated for sideways mounting, I don’t know if it might cause problems or not but at least something you might want to think about.