This Sunrace hub is not an IGH, its a hub for a cassette, the Riese & Müller Birdy Touring has a pure 10s derailleur transmission (while the Pacific Cycles Birdy Touring is a 3x8s with an IGH, quite confusing same model name for different components).

This Sunrace hub+cassette system is meant for small wheels bikes and is derived from the old Shimano Capreo group. Its proprietary, the hub only accept special Sunrace cassettes (with 10s or 11s) and these proprietary cassette can only be mounted on the special Sunrace hub (and maybe on a Shimano Capreo hub ?), mounting requires a special tool (similar to the Shimano cassette and centerlock disc tool but different).

Its very difficult to find these Sunrace special cassettes as spare parts. (same for the hub but there is usually no need to replace it).

The gear inch range provided by this 9-32t cassette with the Birdy Wheel size is excellent and fit the needs of most users.

For those like me who don't like that transmission, the other possibility is to use a SRAM XD or XDR compatible rear hub (the Birdy rear hub is a standard 135mm OLD hub with 5mm quick release) with a Ethirteen or 3T 11S 9-34t or 9-32t cassette same as mounted on the Helix Superlight.