To add an Allowed Domain to all SharePoint sites with an external sharing policy limiting by domain the following PowerShell script can be used.

# Add domain to Allowed Domains - replace DOMAIN2.COM as required

Connect-SPOService -Url https://x-admin.sharepoint.com

# List sharepoint collections
Write-Host "Getting list of site collections..."
$sites = get-sposite -limit All | foreach { Get-SPOSite $_ | select url, SharingDomainRestrictionMode, SharingAllowedDomainList} 

$DomainToCheck = "DOMAIN2.COM"
Write-Host "Checking sharing mode in sites"
foreach ($site in $sites) {
   $sharingmode = $site.SharingDomainRestrictionMode      
   If($sharingmode -eq 'AllowList') {
    $sharingdomainlist = $site.SharingAllowedDomainList
	if($sharingdomainlist -like '*'+$DomainToCheck+'*')
	{
		Write-host $site.url "already contains domain - $sharingdomainlist" -foreground green
	}
	else
	{
		Write-host $site.url "does not contain domain  - $sharingdomainlist" -foreground red
		$newdomainlist = $sharingdomainlist+" $DomainToCheck"
		$newdomainlist = $newdomainlist -replace ",", ' '
		Write-host "Setting to $newdomainlist" -foreground red
		set-sposite -identity $site.url -SharingDomainRestrictionMode AllowList -SharingAllowedDomainList $newdomainlist
	}
   } else {
		Write-host $site.url "not using AllowList" -foreground green
   }
}

It should apply immediately.