Only resolves UAMI in same RG as the hosts

files:
https://github.com/Azure/RDS-Templates/blob/master/ARM-wvd-templates/nestedtemplates/managedDisks-galleryvm.json
https://github.com/Azure/RDS-Templates/blob/master/ARM-wvd-templates/nestedtemplates/managedDisks-customimagevm.json

Part:
"[resourceID('Microsoft.ManagedIdentity/userAssignedIdentities/',parameters('userAssignedIdentity'))]": {}

Issue:
This resolves only UAMI's in the same resource group, not very handy if you have many AVD environments that you would like the use the same UAMI with.

Possible Solution:
do not resolve this resourceid inside the template but rather let the parameter input already be a ResourceID, this way you could even use a UAMI from a different RG or Subscription. This gives a lot more flexibility.
Out of safety consideration, we could even do a check if it's a name or a Resource ID and only resolve it to a ResourceId if its a Name

Start with the parameter and managed identity reference in nestedtemplates/managedDisks-galleryvm.json and nestedtemplates/managedDisks-customimagevm.json. Compare how the current resource ID is resolved, then verify that a user-assigned identity supplied from another resource group or subscription is accepted while existing name-based input remains supported.

Written by the indexing model from the issue text.