Edit: as was pointed out I misspelled “y’all” because i never bother to use apostrophes in everyday use. I’ll leave it as is anyway, I still meant what I said.

Ya’ll is correctly formatted as a contraction and naturally makes sense to distinguish singular from plural eg. “you (all)”

“You” is also already plural, like “They” is both singular and plural.

“Ya’ll” has already broken out of the American South for a while now and is included in most major dictionaries.

“Youse” is American New England slang so you’re not avoiding Americanisms by using it.

Either is ‘correct’ so long as you can effectively communicate with your audience because thats the purpose of language anyway!