There’s a definition independent from 1st/2nd/3rd world [0] and the US’ income seems to clearly make it developed.
Your description of it as a "developing country" is completely inaccurate.
[1] https://www.foreignaffairs.com/world/great-convergence-equal...
As much as the current situation in the US upsets me, anyone who has spent considerable time within the US and outside of the US cannot believe what you've stated as anything other than a trite delusion. The US isn't just the richest country in the world, it has the second highest purchasing power parity per capita in 2026 and historically the highest. The average person in Mississippi is economically better off than the average person in England.
Seems to substantially disagree that the average person in the U.K. is worse off than the average person in the U.S.
It's common to mix up developing/developed and third/first world because they were largely aligned during the cold war. But there is one first-world developing country.