For complex stuff I write Python or Go programs to build manifests, then shell out to kubectl apply. An old example - deploying a multi-instance modded Arma 3 server on k3s: https://github.com/dharmab/homelab-k3s/tree/main/lab
T.b.h. if I were to write a manifest generator, I would still probably commit the thing into a repo and let argo do the rest. Maybe even fiddled around to make the generator into a config-management-plugin ... but that feels like over-doing it.
For complex stuff I write Python or Go programs to build manifests, then shell out to kubectl apply. An old example - deploying a multi-instance modded Arma 3 server on k3s: https://github.com/dharmab/homelab-k3s/tree/main/lab