but I guess it is kind of ridicules that it took about 20 years to make the first one and like 3 years to make the 2nd bigger one
Mmm, maybe we could say that the first Death Star took three years to build also. It is just its "destroyer of world" ultimate weapon that is not fully operational yet at the begining of ANH. Maby the Death Star was a secret base for the Empire for a long time.
I am not a Lucas apologist here, I just try to (desperately) find a solution.