You can 100% have taxes on anarchy.
Anarchy is just about having no hierarchy. You still have rules, you don't have rulers.
People will still want to pull money together for public endeavours.
Discussion
Pooling resources for some useful purpose is called charity or business partnerships, and done voluntarily.
They certainly provide better outcomes - in case of charity, even where gangsterment doesn't or even worsens outcomes.
Only two gangs pool resources coercively - mafia and gangsterment bureaucracy.
Both are as far as can be from anarchy by definition.
If it's proper for them to coerce money, it's certainly proper for a local charity to threaten putting you in a cage if you don't pay your anarchic "tax" to their outcome improvement cause, and do that if you don't agree to their extortion.
I think you are confusing where we are to where we want to be. Because I don't disagree with you in principle, but the implementations possible under this system are not that flexible.
Getting to where we want to be, starts with not wishing armed looting on anyone, and identifying that as an injustice, as in OP.
Those who do wish that upon a minority, deserve the same gangs pilfering them at least proportionally, and usually get robbed more; they don't have the resources to protect themselves.
Whether they are gangsterment members or one of their cronie leeches, anywhere in the "developed world" they are more "rich" than they think.