I did not phrase it clearly: The Birdy as made by the manufacturer was already fine in that regard. I just made it even better.

There is no gravel version of the Birdy that I know of. A shame, really. I’d love to see something like the G-Line. While it is in its 3rd generation and is a great design apparently there has been no further development of the Birdy for several years. Unlike Brompton where the folding bike is the company the Birdy is a keepsake / side show for Riese & Müller: They concentrate on ebikes these days.
One of the founders recently started another company where an electric cargo bike with a built-in reclining chair / bed and a tent construction overhead apparently becomes the perfect glamping vehicle …