By the way, since we discussed 9t cassettes: There is also Campagnolo Ekar: https://www.campagnolo.com/US/en/Mec...s/ekar/gravel2

They have 13-speed 9-36t cassettes among others. That's a good gearing range for the Birdy, assuming you can find a dealer, a 135 mm QR disk hub for their N3W cassette standard, that you accept using drop bars (makes the folded bike bigger), and have a good chunk of extra money. It's something like € 750 for the Ekar parts you need, plus hub and drop bars. Many caveats