I don't own a Birdy myself so was just giving general advice. I would search for "Hylix 31.8 carbon handlebar" on ebay. Hylix makes light stuff for reasonable money and they've been in business for years so maybe safe since no one reports of problems. Going lighter would be big money from Schmolke or the other German boutiques.

Another ebay search for just Birdy results in some light hubs from J&L which also has been in business for years. Can't find any reviews on them though.

This guy added 20" rims but you need skinny tires for it to work. Fold doesn't look much worse.

https://www.foldingtales.com/home/2016/12/29/birdy-long-term-review