If either/both of you are seriously interested in what is rather exotic estate planning, then you'll have to consult a qualified professional who specializes in this area of the tax code. Fundamentally you're asking whether you can go into direct battle and defeat, or at least blunt, U.S. estate tax provisions that have evolved and been refined over a century -- and how to do it. Maybe you can, but it's not easy, certainly won't be cheap, may not be rock solid, and usually won't be durable.
Just as an interesting aside, an estate tax case triggered the U.S. Supreme Court's landmark 2013 ruling in United States v. Windsor that required the U.S. federal government to recognize same sex marriages. Thea Spyer and Edith Windsor married in Toronto in 2007. Spyer died in 2009 and left her entire (and quite large) estate to her wife. There's an unlimited marital exemption between U.S. citizen spouses in the U.S. estate tax code, but the IRS cited Section 3 of the Defense of Marriage Act (DOMA) and refused to recognize their marriage. Since Spyer's estate was valued above the US$3.5 million exemption her estate was allowed in 2009, the IRS assessed estate tax at $363,053. (Looking at the tax rate tables for 2009 it appears to me that Spyer's estate was around US$4.6 million in 2009 dollars. The court records would probably give the exact amount.) Windsor took the case all the way to the Supreme Court and won, and thus she changed history profoundly, forever. So indeed, sometimes estate tax (and broader) victories are possible, even big victories, but not often.
Windsor -- or, more precisely, Spyer's estate, with Windsor as beneficiary -- received a full refund from the IRS. Windsor died in 2017, when the U.S. estate tax exemption was US$5.49 million (since she was a U.S. person). I'm not aware of any information on the tax her estate paid, if any, but my recollection is that Windsor was extremely charitable and donated a lot of money to civil rights causes during her lifetime. My best guess is that her estate fell below the exemption.