Hey Simon,
Thanks for the update! Since your new domain, smartgptsolutions.com, is a federated domain with GoDaddy and can't be set as the primary domain, things are a bit different.
Unfortunately, because it's not a managed domain, you won’t be able to change the base URL for your existing OneDrive and SharePoint to use the smartgptsolutions.com domain directly. Microsoft keeps the onmicrosoft.com domain as the base for these services in such cases.
That said, for sharing files, you might consider creating custom sharing links or use aliases to reflect your smartgptsolutions.com domain in other ways. For new resources in Azure, just keep using the smartgptsolutions domain where possible during setup.
If this workaround doesn’t meet your needs, it might be worth reaching out to Microsoft support for any potential alternatives. I know it’s not an ideal solution, but hopefully, it gives you a direction to work with.
Let me know if you need anything else!