If the goal was simple "equality", much more reasonable than an estate tax is an estate cap. And one much higher than $12 million. Say, up to $100 million inherited by any one individual, or 1/3 may go to a designated heir, and up to $100 million to particular individual.
So, if you had someone with a fortune of $500 million, you can have $166.5 million to a designated heir, and then the remaining $333.5 million has to be divided up between at least 4 other inheritors. For the truly rich, like a Bloomberg with a net worth of $59 billion, that's about $20 billion to whoever continues on the Bloomberg group name, and the remaining $40 billion gets divided up amongst at least 400 individuals.
That's a much more effective way to regulate inheritance to reduce wealth inequality, much more than taking money from a relatively smaller concentration of wealth and power and transferring it to the largest concentration of wealth and power in the country.
This actually distributes power and wealth between a bunch of legitimately independent actors, counteracting a bit the peratio distribution where things naturally accumulate to a 80/20 of an 80/20, while expanding the class of the indepently wealthy with fuck you money to act as counterparts to anything.