NuttX is a POSIX compliant UNIX-like RTOS with a bunch of features:
https://nuttx.apache.org/docs/latest/introduction/about.html
It has been ported (partially?) to the Duo S:
With possible support for SG2002 boards and the 700MHz C906 core:
" * We might run NuttX on the SG2000 Co-Processor (Plus SG2002 with its upsized TPU / NPU)"
Very exciting!