Change top level Sharepoint link

simonforsyth You maybe able do it by injecting custom JavaScript to change the URL on page load with the help of SPFx application customizer for modern experience pages.

Note:

1. DOM manipulation & CSS customizations are not recommended by Microsoft and some of your customization may break if Microsoft changes HTML element id/classes in new release updates.
2. This solution may not work for old classic pages / application pages. For example: site settings page.

Check these threads for more information:

1. How can I include the same JS and CSS files on multiple SharePoint Modern Page? 
2. Hiding Office 365 Ribbon in SharePoint 

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