Modified kfdef.yaml showcasing option 1: Registry path can be taken in global scope & image and tag can be taken from app.yaml per application

KfDef.yaml

apiVersion: kfdef.apps.kubeflow.org/v1alpha1
kind: KfDef
metadata:
  name: kubeflow_app
  namespace: kubeflow
spec:
  repos:
  - name: manifests
    root: manifests-master
    uri: https://github.com/kubeflow/manifests/archive/master.tar.gz
  - name: kubeflow
    root: kubeflow-master
    uri: https://github.com/kubeflow/kubeflow/archive/master.tar.gz
  applications:
  # Istio install. If not needed, comment out istio-crds and istio-install.
  - kustomizeConfig:
      parameters:
      - name: namespace
        value: istio-system
      repoRef:
        name: manifests
        path: istio/istio-crds
    name: istio-crds
  - kustomizeConfig:
      parameters:
      - name: namespace
        value: istio-system
- kustomizeConfig:
      overlays:
      - istio
      repoRef:
        name: manifests
        path: argo
    name: argo        
      repoRef:
        name: manifests
        path: istio/istio-install
    name: istio-install     
  - kustomizeConfig:
  # Update below parameters(*Image & *ImageTag) if any other custom images and tag to be used
      parameters:
      - name: workflowcontrollerImage
        value: workflow-controller
      - name: workflowcontrollerImageTag
        value: v2.3.0      
      - name: argouiImage
        value: argoui
      - name: argouiImageTag
        value: v2.1.0              
      overlays:
      - istio
      repoRef:
        name: manifests
        path: argo
    name: argo
  enableApplications: true
  packageManager: kustomize
  skipInitProject: true
  useBasicAuth: false
  useIstio: true
  version: master
  # Update below parameter(registry) if any other registry to be used
  registry: kubeflowinternalregistry.org