Inbetween steps: different props to see how the engine responds
An APC 9 x 6 brings the RPM up by 1100 to 12100 and gives a power increase from 0,4 to 0,44 hp

Going even lighter with APC 9 x 5 brings RPM to 13000 and raises power to 0.46 hp.

The 9 x 5 gives a slightly better static pull, but chances are, the 9 x 6 will perform better in the air. Remains to be determined.