> I think the biggest injustice is that return of capital isn't taxed at the same rate as income from labor.

Almost there. One step further and you’ll realize the root injustice is that rent is taxed less than either returns to capital or labor.

Compensation for labor and returns to productive risk bearing or entrepreneurship should not be taxed. Rent seeking, where profit is guaranteed disproportionate to investment of labor or capital should be taxed out of existence.

It’s crucial to disambiguate land and capital (in the economic sense: https://en.m.wikipedia.org/wiki/Factor_payments) when speaking about concentration of wealth.