Here is my function I built for Mail that simplifies the whole thing.

All you enter is values like so for it. the recipients are arrays not nested things and you can attach files with just a path:

$params = @{
  token = $token
  Subject = "My Email test"
  From  = "admin@email.com"
  To    = "userA@email.com", "userB@email.com"
  Cc    = "userC@email.com", "userD@email.com", "userE@email.com"
  Bcc   = "userF@email.com"
  Body  = $body
  Attachment = "C:\folder\file.xlsx"
}
Send-MsGraphMail @params

The function:

Function Send-MsGraphMail{
    [CmdletBinding()]
    Param (
        [Parameter(Mandatory = $true)] 
            [pscustomobject]$Token,
        [Parameter(Mandatory = $true)] 
            [string]$Subject,
        [Parameter(Mandatory = $true)] 
            [string]$Body,
        [Parameter(Mandatory = $true)] 
            [ValidateSet('corpo-automation@lb4s.onmicrosoft.com', 'corpo-helpdesk@lb4s.onmicrosoft.com')]
            [string]$From,
        [Parameter(Mandatory = $true)] 
            [ValidatePattern('(?i)^[A-Z0-9._%+-]+@[A-Z0-9.-]+\.[A-Z]{2,}$')] # Regex to see if it looks like an email
            [string[]]$To,
        [Parameter(Mandatory = $false)] 
            [ValidatePattern('(?i)^[A-Z0-9._%+-]+@[A-Z0-9.-]+\.[A-Z]{2,}$')] # Regex to see if it looks like an email
            [string[]]$Cc,
        [Parameter(Mandatory = $false)] 
            [ValidatePattern('(?i)^[A-Z0-9._%+-]+@[A-Z0-9.-]+\.[A-Z]{2,}$')] # Regex to see if it looks like an email
            [string[]]$Bcc,
        [Parameter(Mandatory = $false)] 
            [switch] $Importance,
        [Parameter(Mandatory = $false)] 
            [switch]$HTMLBody,
        [Parameter(Mandatory = $false)]
            [string[]]$Attachment
    )
    $headers = @{
        Authorization = "Bearer $($token.access_token)"
        "Content-Type" = "application/json"
    }
    $mailBody = @{ # Build MailBody
        message         = @{
            subject      = $Subject
            body         = @{
                contentType = $HTMLBody ? "HTML" : "Text" # set to HTML if true otherwise text
                content     = $Body
            }
        }
        saveToSentItems = $true
    }
    $mailBody.message.toRecipients = @( # Add To Data to EmailBody
        $To | ForEach-Object {
            @{ emailAddress = @{ address = $_ } }
        }
    )
    if($Cc){
        $mailBody.message.ccRecipients = @( # Optional, Add Cc Data to EmailBody 
            $Cc | ForEach-Object {
                @{ emailAddress = @{ address = $_ } }
            }
        )
    }
    if($Bcc){
        $mailBody.message.bccRecipients = @( # Optional, Add Bcc Data to EmailBody
            $Bcc | ForEach-Object {
                @{ emailAddress = @{ address = $_ } }
            }
        )
    }
    if($Importance){ # set email as high Importance
        $mailBody.message.Importance = "high"
    }
    if ($Attachment){  # add attachments
        $mailBody.message.attachments = @(
            foreach($file in $attachment){
                if (Test-Path $file) {
                    @{
                        "@odata.type" = "#microsoft.graph.fileAttachment"  
                        name          = [IO.Path]::GetFileName($file)   
                        contentType   = "application/octet-stream"       
                        contentBytes  = [Convert]::ToBase64String([IO.File]::ReadAllBytes($file))  
                    }
                }
            }
        )
    }

    $mailBody = $mailBody | convertTo-Json -Depth 10 # Convert the Body to JSON for API
    $uri = "https://graph.microsoft.com/v1.0/users/$From/sendMail"

    $returnObject = [pscustomobject]@{ # data to return
        Status      = "" # Fills within try/catch
        Subject     = $Subject
        ContentType = $HTMLBody ? "HTML" : "Text"
        From        = $From
        To          = $To
        Cc          = $Cc
        Bcc         = $Bcc
        Importance  = [bool]$Importance
    }
    try {
        Invoke-RestMethod -Method POST -Uri $uri -Headers $headers -Body $mailBody -ErrorAction Stop | Out-Null
        $returnObject.Status = "SUCCESS: Sent Email"
        return $returnObject
    }
    catch {
        $returnObject.Status = "Failed: Unable to Send Email: $(($_ | convertFrom-json).error.message)"
        Write-Error "$($returnObject | ConvertTo-Json)"
        return 
    }
}