To Set Modern Page as a Home Page in SharePoint Online you can use the following Code from SharePoint Online PowerShell:
$siteUrl = "Your Site Url"
$password="Your Username"
$username="Your Password"
$WelcomePageUrl ="Welcome Page Url" #SitePages/WelcomePage.aspx
#Add Reference dlls
Add-Type -Path "C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\15\ISAPI\Microsoft.SharePoint.Client.dll"
Add-Type -Path "C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\15\ISAPI\Microsoft.SharePoint.Client.Runtime.dll"
Import-Module Sharegate
#Set Connectivity Information
write-host "Connecting to SharePoint "$siteUrl
$pwd = convertto-securestring $password -asplaintext -force
$ctx = New-Object Microsoft.SharePoint.Client.ClientContext($siteUrl)
$credentials = New-Object Microsoft.SharePoint.Client.SharePointOnlineCredentials($username, $pwd)
$ctx.Credentials = $credentials
#Set Welcome Page
$web = $ctx.Web
$rootFolder = $web.RootFolder;
$rootFolder.WelcomePage = $WelcomePageUrl;
$rootFolder.Update();
$ctx.ExecuteQuery();