Asset allocation for money you’ll never spend

The way you have phrased this question reveals a hoarder's mindset that is common in traditional personal finance, and unfortunately goes largely unquestioned.

Which is that THE MOST IMPORTANT THING IN THE ENTIRE WORLD BAR NONE IS TO HOARD AS MUCH MONEY AS YOU CAN AND WATCH THE NUMBER GO UP AS HIGH AS IT CAN GO UNTIL YOU ARE DEAD BECAUSE THAT IS HOW YOU WIN LIFE AND NUMBER GO UP IS THE MOST IMPORTANT PRIORITY.

This mindset needs to be beaten senseless because it does not belong in the FIRE community. We are not money hoarders. We are life efficiency experts.

And as if your heirs are going to celebrate you for that. Guess what? They won't. Instead, they will think you were a skinflint who valued number go up money more than relationships.

The better approach is to toss that mindset in the trash and think about how the money can actually be used by the people who are going to be using it and on what time frame(s).

If they are not going to use it, the best approach is to give it to the people who are going to get it anyway, assuming those people can handle money, and if they can't, they'd better be looking at trust arrangements.

From an investment perspective, the money is far better off in Roth accounts that last for the life of the recipient, NOT the giver, which is all you get if you hoard it. And if they are 70, their children are likely in their 30s or 40s, perhaps raising the grandchildren (even longer Roth time), AND COULD REALLY USE THE MONEY NOW, not in 15-20 years when they are either at their highest tax brackets and about to retire themselves. Again, hoarding it now is just dumb and dumber on all accounts. (And now don't start backtracking and start babbling about "they might need the money", because YOU ASSUMED TO BEGIN WITH THAT THEY DON'T AND THAT IS WHAT WE ARE ANALYZING.)

The investment question is actually very trivial and not worth spending much time on. Money should be invested on the appropriate time frame given when it is expected to be used. If nobody is going to use it for a decade or more, 100% equity index funds would be great or Warren Buffet's 90% S&P 500 and 10% t-bills or something. If people are going to use it sooner, that portion should be invested like a retirement portfolio or in cash if the needs are immediate. These principles are invariant as to whose account it is actually in, but as noted, its better off growing in the youngest person's Roth account who is likely to use it. Your tax situations may vary.

This sounds like a perfect post for @UncleFrank to tackle.

Here are my uneducated two cents- the intended use of the $1M completely determines how it should be invested. When I say "you", I am talking to the hypothetical retirees.

1. Maximize Wealth- If the goal is to pass it on and there’s a 15+ year horizon, I’d treat it like accumulation. A simple, low-cost total stock index fund (à la JL Collins) makes sense. The retirees don’t need the money, so volatility is irrelevant. There is enough time to ride out any market downturn so that when you die, your heirs are wiping their tears with crisp $100 bills they have from the inherited $3M+.
2. Maximize Good- Should you instead want to try to maximize your impact on the world rather than inheritance, there are two clean paths. You can donate lump sum now, when dollars arguably do more good than later (the highest-impact problems are cheapest to solve early), or invest it like the retirement portfolio and donate the ongoing gains.
3. Maximize Simplicity- If you want to minimize the number of portfolios and side quests, just treat it like your other $1M.

Another option, if this happens to not be a hypothetical, is to just transfer the excess to me. I could find a use for it.