Here's the thing though, if I'm not already an effective political actor (i.e. someone with a lot of money, or a significant network of people who listen to me, or in a position of control over an institution with proxy access to either of those), the opportunity cost of doing nothing is basically zero.

The assertions that "inaction is a choice" or "political disengagement is a privilege" are coming from a place of naive idealism about the reality of affecting change in modern societies. There's an enormous threshold of activation energy one has to cross to be an efficacious actor against existing large scale power structures, and anything short of it is basically not going to make a difference.

The reality of scale in our world as currently configured is that one private individual could do all the conscientious things - boycott the right things, recycle the right things, vote for the right things - and nothing will change. Even a thousand or a million such individuals randomly distributed doing the exact same things, still wouldn't make a significant difference. Only by acting in coordinated networks would change come about, and it would be driven by the powerful individuals or organizations doing the coordinating, having expended enormous energy to do so. Most people aren't suited to or interested in expending that kind of energy to lead such efforts, and we shouldn't be made to feel guilty for just wanting to go about our lives without trying to "make a difference".