During the 2021 MS Ignite conference, Microsoft announced the ability to rename your default SharePoint Online Office 365 tenant URL. There are quite a few limitations (such as you have to delete the desired name if you have it in another tenant and wait 3 days, sizing limitations, you can only rename a site once every 6 months, any previously deleted sites cannot be restored, and a few others listed here: https://docs.microsoft.com/en-us/sharepoint/change-your-sharepoint-domain-name )
Start-SPOSiteRename is the command AFTER you have added the new URL to the admin portal. You can schedule it a day ahead or further in the future.
Once complete, your old URL will redirect to the new URL. However, I am not sure how long they will honor that redirect, but I assume as long as you have it listed under your tenants URLs it should redirect.