They killed maybe a million at most across half a decade, and much of that number comes later in the war as the situation grew desperate and supply lines were destroyed.

That's wrong by every measurement, including the mass graves scattered across all of Europe, including Romania.

Even going by the numbers that the camp authorities themselves today use it doesn't breach 2 million.

Your thinking of just Auschwitz. The number has since been reduced to somewhere closer to a little over a million for just Auschwitz. The Camp Commandant said that the 3 million Soviet Figure was possible in total deaths, but that executions would have been closer to 2.5. He backtracked and said 2.5 would be too high given the maintenance and logistical needs. Current estimates have revised that figure lower (which is the opposite of the claim by holocaust deniers) and analysis suggest that total deaths at Auschwitz were somewhere over a million.