Hi N2G,
you are asking about a very difficult thing. I already asked about this a lot of times, and everyone has their own way to deal with this problem. Think ahead of time? No, impossible, native speakers don't plan sentences in advance... so I'll tell you what I do, what I learned asking.
I use "they/them/their" when the sex is unknown, unless I need to repeat "they/them/their" a lot of times, or I'm talking about a complex situation and "they/them/their" might be confusing. In such cases, I use "he", or "she", and often point out I don't kow the sex of the person... Example:
I just read a book by Xursufiaa Nugytwery Donnyasta. He, provided he's a male, is a good writer, in my opinion. He wrote a lot of books, and... --- Maybe one day I might find out the author is a female, but I'm now supposing he's a male.
That said, I'm still not completely sure what to do in cases like:
- I have to go now, my cousin needs help.
- What does he want? / What does she want? / What do they want? / What does your cousin want?I use the undelined versions, though, so notice that repeating "your cousin, the author, that person, etc." is a possible option too.