Here is the tip I found to have NFS enabled at boot time, in order to run a nfs-client provisioner in my K3S.
Initially, I went to add a run_cmd with rc-service nfsclient restart in the config.yaml. But without any success.
After many experiment, I found a more robust solution: add a link in runlevels:
boot_cmd:
- "ln -s /etc/init.d/nfsclient /etc/runlevels/default"
HTH