I made some progress:
I added ashmem and binder modules using this script:
https://github.com/choff/anbox-modules
Then Waydroid finished installation correctly.
But the S9xx kernel is still missing PSI to make waydroid start correctly, I tried psi=1 as kernel command-line but PSI is not compiled on this S9xx kernel
https://github.com/waydroid/waydroid/issues/136
I don't know how to add this psi module to my kernel.
Could it be possible to add all those 3 missing modules (ashmem, binder and psi) to the s9xx kernel config file, so waydroid could work directly on armbian S9xx kernel ?
like this one:
https://github.com/armbian/build/pull/4806
CONFIG_ASHMEM=y
CONFIG_ANDROID_BINDER_IPC=y
CONFIG_ANDROID_BINDERFS=y
CONFIG_ANDROID_BINDER_DEVICES="binder,hwbinder,vndbinder,anbox-binder,anbox-hwbinder,anbox-vndbinder"
CONFIG_PSI=y
# CONFIG_PSI_DEFAULT_DISABLED is not set