Thanks for the infos. Is the 9T Capreo cassette still sold? If not, what other brands should I look at?

If I go for a dual chainring and a regular 11T cassette instead, any recommendation for an affordable Hollowtech MTB bottom bracket + crankset combo for use with a Birdy?

Using a ruler to measure the current setup (original Driveline BB and a Sora two-speed derailleur), it looks like ~15mm is needed to move the inner ring far enough for the derailleur to work its magic, so a hollowtech MTB might not be good enough

For ~the price of a new hollowtech bottom bracket + crankset (provided there's a combo that would solve this problem), do you know of a lightweight square taper bottom bracket instead of eg. Shimano's BB-UN300 (330g)?

I have a SuperB 8918 tool handy: Will it work to unscrew the hollowtech Driveline BB?