That's a very good VFD, but this won't solve, your torque at lower RPM, you have a air cooled spindle which has a minimum speed, they are not much good for anything below the minimum RPM of your spindle, they have max torque at the max RPM, so when trying to use them at lower speeds the torque / Hp drops off quite quickly
Water cooled can cut at lower RPM than the air cooled spindles 6,000 is there minimum, but only have a useful cutting range from around 7,500 to 24,000 RPM