FYI, when the ECU decides the loading is within spec and allows a regen, you should hold down the accelerator and brake until the regen initiates. It helps to turn on all electrical consumers like seat heaters, defrost, etc. to increase the load:

I ran it successfully on my old 2012 Passat TDI (that used UDS) and I remember buckling up my seatbelt before it worked, but that may just have been coincidence.