Right now, you’re able to select the appropriate gender marker on your passport without a note from a medical professional (and a passport can be used in the vast majority of situations you’d use a birth certificate) but because the self select gender marker was done by executive action, it’s extremely likely to be reversed when Trump takes office.
Also, having one might be a good idea in general.
Another tip: A passport card is considered as valid as a passport book within the US, but it is much cheaper to obtain. It is conclusive proof of citizenship and an authoritative RealID-compliant identity card, just like a passport book. Passport cards are valid for land and sea travel within North America and the Caribbean. They’re not valid for international air travel (but they can be used for domestic flights).
Edit: Upgrading from a passport card to a passport book is considered a renewal rather than a new application. You can still keep the card if you later want to “renew” it for a book.
This sounds like exactly the type of thing they’ll reverse, either through executive action or pushing through the GOP Congress and Supreme Court. If you can afford it, go for the full US passport.
Let them reverse it then. That is no reason to not get one now.