Re: Blitzkrieg or steady progress?
as per the OP
I go with a mix of the two, basically I blitz their most important economical/recruiting cities; whether I eradicate any field army or not before taking the cities, it depends on the faction I'm playing (as a general rule, cavalry/archery factions I prefer to be on attack so I kill everything before sieging, infantry factions I prefer to defend so I actually wait for field armies to attack me while I'm sieging the settlement).
Once the enemy "heartland" is in my hands I usually move to another target and use smaller armies with new generals to mop up or contain the enemy (depending on if I want to eradicate them or only taking their good settlements and disminish them).
Furthermore, usually I try to roleplay, so sometimes it depends on it (i.e. as dorfs I go for Moria before of any other settlement of the MM)