I've noticed that the only real differences between door types are:
a) piston-powered stone (less than obsidian, End Stone is best) are strongest
b) Iron Doors can be automated
c) oak and iron doors have limited see-through features, and jungle doors are similar in a different arrangement
d) acacia doors are almost completely see-through while still having the same resistance

i know, but i don't want to make such huge things on a servermap atm

Which is why I mentioned the other doors (in place of the Dark Oak door you used). An Acacia door would let you see what's in the space between doors without entering, and is just as durable.