I am aware of some older studies on marginal utility of money for individuals, but they span quite low income/wealth. I have seen some small data that suggest that at the very least the drop in marginal utility is much weaker than previously thought for high wealth brackets. Do you have some pointers to newer data on UHWIs, for example?

I personally have found that there were times when the marginal utility of my additional wealth was growing as I could suddenly do things I could not do before... (while it other times it almost felt like 0)