"Cher" alone doesn't mean anything. At all. "Cher [name]" exists but it's just to open letters.
"Mon cher" is "my dear" but in the sense of "oh my dear Mr Smith fancy running into you at this barber shop"
You wanna be fancy-old-timey-loving, at least go with "très cher" ("dearest", lit. "very dear"). It's still stilted but at least it's personal.
You wanna get really personal, with a phrase that's still actually used this century, the word you're looking for is "chéri" ("darling" but a bit more relaxed, lit. "cherished", "beloved"). Potentially "mon chéri", which is the exact equivalent of "my darling".
Also this is mostly about FrUK but if speaking to a woman you go "ma chère", "très chère", "chérie" and "ma chérie" because this language is so gendered help us we're dying au masculin neutre.
Merci very much.
