I’m definitely not an economist, and unlikely to ever have much experience with an upper middle class income, but I’ll throw in the two pennies I can spare:
I think it’s important to note the difference between an UMC lifestyle and being (economically) upper middle class.
The lifestyle might well include a large home, a fancy car or two, private school, hired help, and a couple of high end vacations every year. Nouveau riche.
But that’s not the economics. You might well be a high earner, happily driving your 15 year old Subaru home to your modest bungalow every day. Spending and earning are very different functions.
(The OP reminded me of a set of questions from my daughter as she completed a homework assignment for Econ 1101 last semester. She was instructed to figure out the net worth of a family, and compare it to income, so I was the obvious source to ask nosy financial questions. And we are struggling. Disabled husband, two kids in college, two in elementary school. We aren’t able to help the big kids much, but we do pay for phones and car insurance, and provided both with reasonable used cars.
However, because we have a very modest amount of debt - currently about $5000 remaining on the mortgage - and don’t mind driving an old vehicle, if it’s maintained, our net worth puts us ahead of the majority of Americans. Yeah, it’s a mobile home, but it’s on almost 5 acres of land that is appreciating fast. Yeah, both trucks are old, but they’re paid off. No, the furniture isn’t fancy, but none was purchased on credit. Our children don’t attend fancy schools, but we intentionally bought a home with access to good public schools, and a good university plus a technical school to take advantage of state funded dual enrollment for high school kids. We live a lower/lower middle class lifestyle, but I make financial decisions that are advantageous.)
Lifestyle, earnings, and absolute wealth are very different things, and I’m not sure that one can describe a class without factoring in all three.