With DC motors the current is proportional to the torque, so you want a variable constant current sink circuit to do programmable braking. This can be completely passive and analog, or powered and switch-mode using a MOSFET and a dump resistor to dissipate the heat.
For a motor without gearing torque is closely related to the size (volume) of the rotor. You can't indepedently choose torque and size unless you can have gears.
20rpm is very slow for a motor (4000 rpm is typical), which also suggests gearing would be a good idea, however a reduction ratio of 200:1 which is back-drivable isn't a viable option, so that's completely out. Just how little drag torque do you want when not actively braking?
20rpm is ~2 rad/s and at 6Nm that's 2 x 6 = 12 watts to dissipate, which isn't too problematical, a medium sized heatsink should be adequate.
BTW with only a small gear ratio (to allow low resistance backdriving), the motor will be spinning slowly (compared to its ratings), and the winding resistance may limit the current too much - you need to know the motor constant (volts per rpm) and the winding resistance to figure out if this might be the case.