One idea to add a slight speed-up mechanic to siege for the attacker would be to add sapping.
With the current changes it seems that siege is highly weighted in favour of the defender, which I think is pretty thematically accurate?
However, maybe there could be a new skill to buy, a new credo or some way to add in Sapping. A player could maybe dig a pit and light a fire with a special kind of construction and work bellows or something against a wall. Maybe instead of doing direct damage and being too close to wallbashing it could add an aoe sort of bonus to siege weapons that hit within a given area?
It might be easy to defend since all you have to do is attack a player to make it stop but it would get people out of their walls and it could be started again instantly.
Just makes the idea of siege more interactive, like instead of waiting for drying times and dealing with events that only happen every X amount of hours (repair siege engine, destroy siege engine, or w/e) something like sapping could happen the entire time and make it all more engaging.