azure bicep function app with event grid trigger from storage account - how to create a function resource for event grid subscription?

Question

I am trying to deploy FunctionApp with an Event Grid Trigger, upon a blob creation within Storage Account. I can succesfully create FunctionApp, Storage Account, as well as Event Grid Topic, however I am struggling with creation of a Function resource (I need that for the Event Grid Subscription). What could be wrong in the template below?

modules/FunctionApp/Function.bicep:

@allowed([
  'http'
  'eventgrid'
])
param triggerType string = 'http'

param functionAppName string


resource functionApp 'Microsoft.Web/sites@2022-09-01' existing = {
  name: functionAppName
}

// var functionApphref = 'https://${toLower(functionAppName)}.azurewebsites.net'

var functionName = (triggerType == 'eventgrid') ? 'EventGridTrigger' 
: 'HttpTrigger'


resource function2inModule 'Microsoft.Web/sites/functions@2022-09-01' = if(triggerType == 'eventgrid') {
  parent: functionApp
  name: functionName
  // location: location
  properties: {
    // script_href: '${functionApphref}/admin/vfs/home/site/wwwroot/function_app.py'
    // test_data_href: '${functionApphref}/admin/vfs/tmp/FunctionsData/${functionName}.dat'
    // href: '${functionApphref}/admin/functions/${functionName}'
    // config: {}
    language: 'python'
    isDisabled: false
  }
}

In the above template you can see some lines commented out - my intention (for now) is to skip them and just create a "placeholder function" under Function App for the Event Grid Subscription. (I hope that's possible?). For now, I am planning to deploy application code later with "Deploy to Function App..." in VSCode.

This is the main.bicep (relevant bits):

// storage account for data
module storageAccount4data 'modules/Storage/StorageAccount.bicep' = {
  name: storageAccount4dataName
  params: {
    location: location
    storageAccountName: storageAccount4dataName
  }
}

// blob service
module blobServices 'modules/Storage/BlobService.bicep' = {
  name: 'default'
  params: {
    blobServiceName: 'default'
    storageAccountName: storageAccount4data.outputs.dtls.name
  }
}

// container
module container 'modules/Storage/Container.bicep' = {
  name: containerName
  params: {
    storageAccountName: storageAccount4data.outputs.dtls.name
    blobServiceName: blobServices.outputs.name
    containerName: containerName
  }
}


var functionApp2Name = '${funcAppNamePrefix}-2-preProcAndWriteToDB'

module functionApp2 'modules/FunctionApp/FunctionApp.bicep' = {
  name: functionApp2Name
  params: {
    location: location
    functionAppName: functionApp2Name
    functionStorageAccountName: storageAccounts4func[1].outputs.dtls.name
    // triggerType: 'eventgrid'
  }
}


// event grid resources
resource systemtopic 'Microsoft.EventGrid/systemTopics@2023-06-01-preview' = {
  name: eventGridTopicName
  location: location
  identity: {
    type: 'SystemAssigned'
  }
  properties: {
    source: storageAccount4data.outputs.dtls.id
    topicType: 'Microsoft.Storage.StorageAccounts'
  }
}
// the above deploys without any problems


// This is the problematic part:
module function2 'modules/FunctionApp/Function.bicep' = {
  name: 'function2Name'
  params: {
    functionAppName: functionApp2.outputs.functionAppName
    triggerType: 'eventgrid'
  }
}

// Later aim to create the following resource:
// resource eventsubscription 'Microsoft.EventGrid/systemTopics/eventSubscriptions@2023-06-01-preview' = {
//   parent: systemtopic
//   name: functionApp2.name
//   properties: {
//     destination: {
//       properties: {
//         resourceId: '${functionApp2.outputs.functionAppId}/functions/EventGridTrigger'
//         maxEventsPerBatch: 1
//         preferredBatchSizeInKilobytes: 64
//       }
//       endpointType: 'AzureFunction'
//     }
//     filter: {
//       subjectBeginsWith: '/blobServices/default/containers/${containerName}/blobs/'
//       subjectEndsWith: '.json'
//       includedEventTypes: [
//         'Microsoft.Storage.BlobCreated'
//       ]
//       enableAdvancedFilteringOnArrays: true
//     }
//     labels: []
//     eventDeliverySchema: 'EventGridSchema'
//     retryPolicy: {
//       maxDeliveryAttempts: 30
//       eventTimeToLiveInMinutes: 1440
//     }
//   }
// }

The error I get is:

Encountered an error (ServiceUnavailable) from host runtime.
(Target: /subscriptions/{}/resourceGroups/{}/providers/Microsoft.Resources/deployments/function2Name)

Any idea what I am doing wrong?