Hydrazine is liquid; an equally toxic solid would be much less dangerous to work with.

The A340-600 has four engines, not six. But the point is still true with, say, a Boeing 787. Here are the figures for the 787-9: maximum takeoff weight: 560,000 lb; maximum landing weight: 425,000 lb [1]. In case of an emergency the pilot has to get rid of fuel before landing if the airliner is above maximum landing weight.

[1] Figures given in pounds on wikipedia, the kg version looks like a "dumb" conversion: https://en.wikipedia.org/wiki/Boeing_787_Dreamliner#Specific...

Fundamentally, landing speeds are lower than takeoff speeds. To travel at the same (low) speed and rate of descent with more weight means getting more lift at the same speed. That is done by increasing angle of attack or adding flaps. There is a limit to both.