It depends of what type of cassette you want to mount.

For a small wheel bike and without a bigger than usual chainring (60 or 65t that anyway isn't compatible with the new Birdy chain tensionner) a cassette with a smaller cog of 9t or 10t is needed (as mounted by R&M on the Birdy Touring).

The classic Shimano/SRAM freewheel body is limited to 11t as smallest cog.

R&M was using the now discontinued Shimano Capreo 9s system and is now using a Sunrace system (R&M use the 10s one, there is also a 11s)..

Another option that I have chosen is to use the SRAM XDR system.

Dropbox is not safe, can you post it somewhere else ?

For the Birdy disc front hub, there are indeed some options, also Hubsmith and Ridea but not easy to really buy.