The gender of the name "Sabine" is not obvious to me. I wish English had a good gender neutral pronoun. "He or she" is too formal sounding. "They/their/them" is too informal sounding.
I wonder why you feel that singular they is informal? There's nothing wrong with the construction. It's attested as far back as 14th century. And, as you mention, it certainly fills a void in the language.
https://en.wikipedia.org/wiki/Singular_they