Does someone know what's actually the fastest/most convenient way of getting from Halong to Mong Cai to Dong xing to Nanning ?

There is bus service between Halong Bay and Mong Cai. It is around 200km between and the bus duration shall be around 3.5h. But I am not sure about the information. You may consult further.

You can cross the border from Mong Cai to Dongxing. Then take local bus to your destination.

At Dongxing Bus Station, there are direct buses to NN city and the bus duration shall be around 3.5h.