Export Mailboxes to PST in bulk, but in batches

Dear Experts, I'm taking over a client that has recently migrated to a new EXCH 19 installation which went smoothly but is cluttered with old and unused mailboxes which belong to disabled users.
I have identified said mailboxes rather easily using the following script:

$Mailboxes = Get-Mailbox -ResultSize Unlimited | where {$_.RecipientTypeDetails -eq 'UserMailbox'}
$Disabled=@()

Foreach ($Mailbox in $Mailboxes) {
    if((Get-ADUser -Identity $Mailbox.SamAccountName).Enabled -eq $False){
        $count += 1
        #$Disabled += Get-MailboxStatistics $Mailbox.SamAccountName | Select -Property DisplayName,TotalItemSize,samaccountname,alias
        $Disabled += [pscustomobject]@{    
            "Username" = $Mailbox.SamAccountName
            "Alias" = $Mailbox.Alias
            "Display Name" = (Get-MailboxStatistics $Mailbox.SamAccountName).DisplayName
            "Total Item Size (MB)" = (Get-MailboxStatistics $Mailbox.SamAccountName).TotalItemSize.Value.ToMB()
        } 
    }    
}
$Disabled | Export-Csv -Path $env:userprofile\desktop\DisabledADUserwithMailbox.csv -NoTypeInformation -Encoding UTF8
Write-Host "Found" $count "Mailboxes of Disabled Users"

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So far is the easy part.
What I'm looking to do is to modify said script so that for every mailbox's samaccountname, The script will check AD Extention attribute Numer 10 - if it has the word "Exported" in it, skip said mailbox - otherwise, export the mailbox to a PST and after completion edit the Extention Attribute Number 10 to say "Exported"

Also, I don't want to to immediately export all mailboxes found as it is over 1K mailboes and I fear it will kill the storage's IOpS capacity, so I'd like to somehow limit it to 5 simultaneous exports while the rest wait in a queue if that makes sense