I’m a 12th grade teacher. The only grammatical issues that come up with pronouns is pronoun-antecedent agreement and subject-verb agreement.
Nikki Hiltz won their first US title last weekend. <—- In the past, most would say that this is a pronoun-antecedent agreement error since “Nikki Hiltz” is a singular proper noun and “their” is a plural pronoun. However, using a plural personal pronoun for one person has been acceptable usage in casual spoken English for long enough that most English teachers do not view it as an error.
Subject-verb agreement with the plural subject pronoun “they” follows the existing rule that it must agree in number with the verb.
”They are” “They were” not “They is” or “They was” even though “They” can sometimes be agreeing with a singular antecedent.
This is similar to the confusion some people have with a word like “each,” which is a singular noun that many mistake for being plural.
“Each of the runners lines up at the start.” That is correct.
if you really can’t bring yourself to use the preferred pronouns for a person, then rewrite the sentence:
”Nikki Hiltz became a U.S. champion for the first time last week.”
Happy to help more if neeeed.