What would be the correct spelling/hyphenation for "upper mid-tier"?

Question

The phrase is for referring to a noun "company" such as in the sentence: "I bought the upper mid-tier company". (Meaning a company that is middle tier but slightly higher and not quite top-tier). I am questioning if it is correct to use "mid-tier" as a compound adjective with a hyphen.

"Mid-tier" seems to be commonly used as a word and referred to in dictionaries with hyphens, but it is occasionally spelled "mid tier" in online publications and style guides seem to support that too. I've looked at style guides on using hyphens which say if a word after "mid" is not capitalized or a number it should be joined without a hyphen, such as "midsummer". So which is correct? And how does "upper" act as a part of speech? Is it an adverb modifying the "mid" or "mid-tier"?

Answer 1

Styles vary, but my preference would be to copy what I would do for upper middle class, similar to Wikipedia's

The upper middle class in Britain broadly consists of people who were ... examples of upper-middle-class people ...

so without hyphens for the noun but with two hyphens for the adjective. Despite Wikipedia's American equivalent only using one hyphen for the adjective, as in

In sociology, the upper middle class of the United States is the social group ... Upper middle-class parents expect their children ...

it seems from nGrams that the two-hyphen style may now be more common there too