How to correctly access v2.0 of REST API in PowerShell for mailbox management - powershell

Question

We have a PowerShell script which checks a service mailbox and updates our wallboards with information, then moves the relevant emails to the deleted folder. This was set up under API v1.0 which is now deprecated and we're needing to get it changed for version 2.0.
Unfortunately, this has led to a lot of confusion and a lot of scratched heads which isn't too fun. This has occurred mainly around getting an oAuth2 token then feeding it into the request for the mailbox.
So far, we've managed to get the Azure AD app registered and it's providing an authentication token, but that's coming back as unauthorised from Microsoft Graph.
# tenantID, clientSecret and clientID not here.
# The resource URI
$resource = "https://graph.microsoft.com"
# Your Client ID and Client Secret obainted when registering your WebApp
$redirectUri = "http://returnuri"
# UrlEncode the ClientID and ClientSecret and URL's for special characters
$clientIDEncoded = [System.Web.HttpUtility]::UrlEncode($ClientID)
$clientSecretEncoded = [System.Web.HttpUtility]::UrlEncode($clientSecret)
$redirectUriEncoded = [System.Web.HttpUtility]::UrlEncode($redirectUri)
$resourceEncoded = [System.Web.HttpUtility]::UrlEncode($resource)
$scopeEncoded = [System.Web.HttpUtility]::UrlEncode("https://outlook.office.com/user.readwrite.all")
$body = #"
client_id=$ClientID
&scope=https%3A%2F%2Fgraph.microsoft.com%2F.default
&client_secret=$clientSecretEncoded
&grant_type=client_credentials
"#
$userid = 'userID'
$accessToken = Invoke-RestMethod "https://login.microsoftonline.com/$tenantID/oauth2/v2.0/token" -Method Post -Body $body
#
#$cred = $(Get-Credential)
Invoke-RestMethod -Uri "https://graph.microsoft.com/v1.0/users/$userid/messages" -Headers #{Authorization = "Bearer $($accessToken.access_token)"} -Credential $cred
Expected results: access to the mailbox.
Actual results:
Invoke-RestMethod : The remote server returned an error: (401) Unauthorized.
At line:32 char:1
+ Invoke-RestMethod -Uri "https://graph.microsoft.com/v1.0/users/$useri ...
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ CategoryInfo : InvalidOperation: (System.Net.HttpWebRequest:HttpWebRequest) [Invoke-RestMethod], WebException
+ FullyQualifiedErrorId : WebCmdletWebResponseException,Microsoft.PowerShell.Commands.InvokeRestMethodCommand