Hey Martin,
No need to stop as long as things remained unclear or help is useful for you 
The braking resistor does exactly this. It helps not only if you want to quickly reach a full standstill but also allows for faster slowing down on speed changes.
I do not know if you can add a braking resistor to a Huanyang VFD, I read that some models are not prepared for it, but every VFD has some built-in brake circuit with a minimal brake resistor anyway. On my VFD you can attach one of two recommended, differently sized braking resistors, which allow for more braking torque which means decreasing the possible spindle decelaration time.
A spindle is an induction motor, and such a motor, when its power is cut, immediately acts as an induction generator and produces power. The chopper circuit inside the VFD consumes the power that the motor produces and transforms it into heat in the braking resistor, thereby slowing the motor down. The smaller the resistor value in ohms is, the more power it will dissipate into heat, but the higher the resistor’s rated power has to be and the bigger it is and the hotter it gets (you can burn your fingers on it).
You can choose the time for the deceleration by a setting in the VFD.
As for the Emergency Stop and/or Switching off, the updated IEC 60204-1 standard explicitly states that the activation of the STOP function of a VFD may be regarded as a category 0 stop. That means “Safe Torque Off” (STO), the spindle is made powerless and therefore cannot exert any torque.
Category 0 Stop means uncontrolled stop by immediate power removal to the spindle. However this also means that because of the spindle being disconnected from power, the drive cannot generate any brake torque and the spindle is left running free (coasting).
What we would want in most cases is not free-run (coasting) of the spindle but reaching a controlled standstill of the motor (decelerating) within a short time, and only after that, cutting spindle from power (“Safe Torque Off”). This is what Category 1 Stop means. Since it reaches “Safe Torque Off” too, it is regarded as a valid Emergency Stop mode too (as well as Category 0 stop is).
As for STOP command in a VFD, you can program the behaviour of what it does on STOP, whether the spindle is left coasting or decelerated within a given time.
Note that if you set the deceleration time shorter than your braking resistor can achieve, this could lead to a VFD trip, which would abort the braking and make the spindle powerless immediately, with no regard whether it was at standstill already or it is still coasting.
But the important thing is that it is clear that when you give the STOP command, finally the spindle reaches the powerless state. It does it anyway.
Okay, this is already good. But I do not understand why you don’t simply turn the VFD on at the same moment you turn the rest of the CNC machine on. This is the usual way to switch the VFD on.
Also what is when you put the router switch of Satoer Control Panel in “auto” mode? If you use the “router” relay of Satoer Control Panel to switch the VFD on and off, it will be switched on and off everytime your g-code program encounters a M3/M4 rsp. M5 command. This is not what a VFD likes. In the manual of my VFD it says:
Do not turn on the power and then turn it off again more than once every 3
minutes. Doing so may damage the Inverter.
Also the M3/M4 and M5 commands in your program would already have been received by the VFD from the Controller, via the ModBus/RS-485 connection! It would make no sense for a VFD to receive a M5 (STOP) command from the Controller, and be switched off too.