By default, all Unicode letters and numbers are allowed when you enable unicode_usernames. Characters with a macron are allowed Unicode characters as well.
If you want to restrict the Unicode characters to characters with a macron, then I’d set the allowed_unicode_username_characters site setting to the following value:
[āǟǡǣḇc̄ḏēḕḗḡẖīḵḻḹm̄ṉn̄ōṓṑȫǭȭȱr̄ṟṝṯūǖṻȳẕ]
If you would like to allow only a subset of those characters, remove everything you don’t need from the list of characters.
Apostrophes aren’t a letter, so they aren’t allowed, and it’s not possible to add them to an allowlist. The reason is that allowing apostrophes in usernames could break functionality like mentions or quotes.