"Hors means outside or except."

The first use case for this preposition is "l’exclusion du lieu." i.e. "exclusion from a place" but it can also mean excepté (see Wiktionnaire):

Ils y sont tous allés, hors deux ou trois.
Hors cela, je suis de
votre sentiment.

Here I would use the "locution prépositive" à part instead, because hors feels formal, archaic and may even be completely outdated in such a context. There are many use cases where you have hors in a composition with a noun to mean either outside a place or outside the scope of a thing.

"I wonder if hors mis can also mean except."

Since the 16th century (LBU14 § 259 H6) there is indeed a set composition from the past participle "hors mis" (a "lexicalisation du syntagme hors mis, participe passé de mettre hors « exclure »", TLFi), and that’s the preposition hormis which means excepté, sauf:

Il y a remède à tout, mon enfant, hormis à la mort (Mérimée, in the
TLFi)

The thing is even though we see both hors and hormis can share that excepté meaning, we still can’t explain why in the last example I can’t see hors ever being used whereas I would think hormis works just fine with the first two examples. The latter is really the one used for excepté. You can forget hors mis with two words, which is out of usage, and focus on hormis.