I've attempted to add the line:

Write-Host $ListItems | ConvertTo-Json

However this doesn't output the contents of the objects themselves, and results in the following:

Is there potentially a way I could amend this to output the fields associated with the files instead (the list field contents)?

#Clear Error Log
$Error.Clear()

Import-Module 'C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\16\ISAPI\Microsoft.SharePoint.Client.dll'
Import-Module 'C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\16\ISAPI\Microsoft.SharePoint.Client.Runtime.dll'
Import-Module "c:\Program Files\Common Files\microsoft shared\Web Server Extensions\16\ISAPI\Microsoft.SharePoint.Client.Taxonomy.dll" 

Write-Host ""
write-host "## SharePoint Online List to JSON converter ##" -ForegroundColor DarkYellow

#Connect to MSOL and Exchange Online. Comment out the MSOL session bits to ignore the exchange bits
$MSOLSession = $Null
$PSCredential = $Null
$0365Credential = $Null
#Determine Enviroment
if ($Env:COMPUTERNAME -eq "CLIENT" -and $Env:USERNAME -eq "CLIENT") {
    write-verbose "Azure Automation Services Environment Detected"

    #use Azure Auotmation Services Stored Credentials
    $PSCredential = Get-AutomationPSCredential -Name Office365AzureAdmin
    $0365Credential = Get-AutomationPSCredential -Name ExchangeOnlineAdmin        
} else {
    write-host "Local Environment Detected"
}

$AdminName = [Environment]::UserName
$PCName = $Env:COMPUTERNAME
 
$AdminUsername = ""
$AdminPassword = $null

if ($AdminUsername  -eq "")
{
    Write-Host "Username couldn't be detected" -ForegroundColor Red
    $AdminUsername = Read-Host "Enter username:"
}

$FileName = $AdminName + $PCName  
$CredsFile = ".\EncryptedPasswords\$FileName-O365Password.txt"
$FileExists = Test-Path $CredsFile

if  ($FileExists -eq $false) {
    Write-Host "Credential file not found. Enter password for $AdminName :" -ForegroundColor Red
    Read-Host -AsSecureString | ConvertFrom-SecureString | Out-File $CredsFile
    $AdminPassword = get-content $CredsFile | convertto-securestring
            
} else {
    Write-Host 'Using your stored credential file' -ForegroundColor Green
    $AdminPassword = get-content $CredsFile | convertto-securestring
}

#Connect-MsolService -Credential $PSCredential
$siteroot = Read-Host "Enter site root (ex https://test.sharepoint.com):" 
$sitesubsite = Read-Host "Enter subsite (ex /sites/teamsite):" 
$site = $siteroot + $sitesubsite
$ListName = Read-Host "Enter list title (ex 'Contacts list'):" 

#Get the Client Context and Bind the Site Collection
$context = New-Object Microsoft.SharePoint.Client.ClientContext($site)

#Authenticate
$credentials = New-Object Microsoft.SharePoint.Client.SharePointOnlineCredentials($AdminUsername, $AdminPassword)
$context.Credentials = $credentials

#Lookup Source Address 
$rootWeb = $Context.Web
$List = $rootWeb.lists.getByTitle($ListName)
$fields = $List.Fields;
$ListItems = $List.GetItems([Microsoft.SharePoint.Client.CamlQuery]::CreateAllItemsQuery())

#Load the List
$Context.Load($rootWeb)
$Context.load($List)
$Context.Load($ListItems)
$context.Load($fields)
$Context.ExecuteQuery()

#Clear Error Log
$Error.Clear()

Import-Module 'C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\16\ISAPI\Microsoft.SharePoint.Client.dll'
Import-Module 'C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\16\ISAPI\Microsoft.SharePoint.Client.Runtime.dll'
Import-Module "c:\Program Files\Common Files\microsoft shared\Web Server Extensions\16\ISAPI\Microsoft.SharePoint.Client.Taxonomy.dll" 

Write-Host ""
write-host "## SharePoint Online List to JSON converter ##" -ForegroundColor DarkYellow 


# Constant settings, leave blank to specify at runtime
$siteroot = ""
$sitesubsite = ""
$ListName = ""
$AdminUsername = ""

#Connect to MSOL and Exchange Online. Comment out the MSOL session bits to ignore the exchange bits
$MSOLSession = $Null
$PSCredential = $Null
$0365Credential = $Null
#Determine Enviroment
if ($Env:COMPUTERNAME -eq "CLIENT" -and $Env:USERNAME -eq "CLIENT") {
    write-verbose "Azure Automation Services Environment Detected"

    #use Azure Auotmation Services Stored Credentials
    $PSCredential = Get-AutomationPSCredential -Name Office365AzureAdmin
    $0365Credential = Get-AutomationPSCredential -Name ExchangeOnlineAdmin        
} else {
    write-host "Local Environment Detected"
}

$AdminName = [Environment]::UserName
$PCName = $Env:COMPUTERNAME
$AdminPassword = $null

if ($AdminUsername  -eq "")
{
    Write-Host "Username couldn't be detected" -ForegroundColor Red
    $AdminUsername = Read-Host "Enter username:"
}

if ($siteroot -eq "")
{
    $siteroot = Read-Host "Enter site root (ex https://test.sharepoint.com):"
}

if ($ListName -eq "")
{
    $ListName = Read-Host "Enter list title (ex 'Contacts list'):" 
}

if ($sitesubsite -eq "")
{
    $sitesubsite = Read-Host "Enter subsite (ex /sites/teamsite):" 
}

$FileName = $AdminName + $PCName  
$CredsFile = ".\EncryptedPasswords\$FileName-O365Password.txt"
$FileExists = Test-Path $CredsFile

if  ($FileExists -eq $false) {
    Write-Host "Credential file not found. Enter password for $AdminName :" -ForegroundColor Red
    Read-Host -AsSecureString | ConvertFrom-SecureString | Out-File $CredsFile
    $AdminPassword = get-content $CredsFile | convertto-securestring
            
} else {
    Write-Host 'Using your stored credential file' -ForegroundColor Green
    $AdminPassword = get-content $CredsFile | convertto-securestring
}

#Get the Client Context and Bind the Site Collection
$context = New-Object Microsoft.SharePoint.Client.ClientContext($siteroot + $sitesubsite)

#Authenticate
$credentials = New-Object Microsoft.SharePoint.Client.SharePointOnlineCredentials($AdminUsername, $AdminPassword)
$context.Credentials = $credentials

#Lookup Source Address 
$rootWeb = $Context.Web
$List = $rootWeb.lists.getByTitle($ListName)
$fields = $List.Fields;
$ListItems = $List.GetItems([Microsoft.SharePoint.Client.CamlQuery]::CreateAllItemsQuery())

#Load the List
$Context.Load($rootWeb)
$Context.load($List)
$Context.Load($ListItems)
$context.Load($fields)
$Context.ExecuteQuery()

Write-Host "List items are"
foreach ($item in $ListItems)
{
    Write-Host "----------------------------------------------"
    Write-Host "Display Title: " $item["Title"]
    Write-Host "Setting:" $item["Setting"]
    Write-Host "File Description:" $item["File Description"]
    Write-Host "Health Area:" $item["Health_x0020_Area"]
    Write-Host "Resource Type: " $item["Resource_x0020_Type"]
    Write-Host "Currently active: " $item["Currently_x0020_active_x003f_"]
    Write-Host "External file path: " $item["External_x0020_file_x0020_path"]
    Write-Host "----------------------------------------------"
}