Hi @Zoey Liang,
Thank you for posting in this community.
1.Hide the Sync button.
Please click on the gear in the upper right corner ->Library Settings->More library settings->Advanced settings->set Offline Client Availability to no.
2.Add a shortcut to Onedrive.
I noticed that you already have a shortcut to OneDrive button in your document library.
According to this article, this feature isn't available for individual files. Only can add a shortcut for folders.
The method of adding A is also provided here for your reference:
You can use PowerShell script to enable the Add shortcut to OneDrive feature.
If you are new to SharePoint Online Management Shell, you can refer to this article for a quick start:
Get started with SharePoint Online Management Shell
Here is the PowerShell script:
#Connect to your SharePoint Online
Connect-SPOService "https://yourdomain-admin.sharepoint.com"
#Disable "Add Shortcut to OneDrive"
Set-SPOTenant -DisableAddShortCutsToOneDrive $False
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