There is no direct tenant settings to remove Sync icon from all sites or newly created site.
PowerShell to Remove Sync Button in all sites of one Site Collection(you need to look through all site collections to meet your requirement):
#Load SharePoint CSOM Assemblies
Add-Type -Path "C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\16\...
Custom master pages are currently not supported for "modern" pages.
You can find supported/unsupported customizations in below documentations:
Customizing "modern" site pages
Customizing "modern" team sites
Branding your SharePoint site
There is no SharePoint out of the box option to hide office 365 ribbon at the top.