अष्टाध्यायी is definitively the way to learn, I would even say that MUST be the only way to learn, someone created this document and posted online, look at all the lopa sutras, pay closer attention to siddhanta Kaumudi and Laghu siddhanta kaumudi numbers.
There is no way one can related all this - the only resort is to memorize blindly.
In siddhanta kaumudi - Halantyam is 1 and then you have to wait for another 27 sutras to read Upadesheanunasika it. However when you read halantyam you would be scratching your head saying where did it come from.
Its a pity that people left अष्टाध्यायी and adopted kaumudi so widely.