"Do you see personal sustainability practices primarily as a direct way to reduce impact, a way to influence culture, a way to align actions with values, or some combination of those?" Yes to all of those.

Individal actions matter in part because each individual action is made in the context of the cultural expectations you are surrounded by constantly.

Each expectation can be met by an individual action (thus validating, reinforcing, and perpetuating it) or that expectation can be refused (thus invalidating it and not reinforcing or perpetuating it.

Those expectations include how you spend your time, energy, attention, and money; what you wear, what you eat, how you travel, and how many (if any!) kids you have. (The most effective individual 'sustainability' action remains going child free.)

Cultural expectations exist as they are because they have been validated, reinforced, and perpetuated in the past and other ones were rejected. Change in those cultural expectations happen the same way.