When you first unlock your bankai, whether it would be with the long or quick method, it wouldn't be at its full power already.
But the long method would give you a more advanced bankai, requiring you to do less in order to get at its full power.
Upgrading your bankai would be achieved through fighting him more times. The stat modifier from the long method wouldn't change (zan has 0.7x your stats) but for the quick method it would progressively go down (first fight after the unlock would be 1.2x, then 1.1x, then 1x etc...) Long method should start with a 50% completed bankai, and quick method with a 20% completed bankai. Each fight would increase the completion by 10%
After fully unlocking your bankai, you could have your bankai's blade broken (has to be the bankai blade not shikai or regular zanpakuto). After your bankai's blade has broken you could have access to a quest to go see the squad 0. From there you'd have to do a questline which results in a choice:
-Stronger bankai
-True bankai
stronger bankai would mean that you get the same bankai abilities, but stronger, so it would depend on the ability, but it could be more range, more damage or stronger effects.
True bankai would completely change your bankai, both visually and in abilities, the abilities would be "evolved" variants of their regular bankai counterpart, so you could add another component to your abilities (for example you have an aoe freezing move, you could add a buff component to yourself)
When you first unlock your bankai, whether it would be with the long or quick method, it wouldn't be at its full power already. But the long method would give you a more advanced bankai, requiring you to do less in order to get at its full power.
Upgrading your bankai would be achieved through fighting him more times. The stat modifier from the long method wouldn't change (zan has 0.7x your stats) but for the quick method it would progressively go down (first fight after the unlock would be 1.2x, then 1.1x, then 1x etc...) Long method should start with a 50% completed bankai, and quick method with a 20% completed bankai. Each fight would increase the completion by 10%
After fully unlocking your bankai, you could have your bankai's blade broken (has to be the bankai blade not shikai or regular zanpakuto). After your bankai's blade has broken you could have access to a quest to go see the squad 0. From there you'd have to do a questline which results in a choice: -Stronger bankai -True bankai
stronger bankai would mean that you get the same bankai abilities, but stronger, so it would depend on the ability, but it could be more range, more damage or stronger effects.
True bankai would completely change your bankai, both visually and in abilities, the abilities would be "evolved" variants of their regular bankai counterpart, so you could add another component to your abilities (for example you have an aoe freezing move, you could add a buff component to yourself)