Mac shared as a Time Machine backup destination
You specify a folder on the Server Mac Pro to be a shared folder, then still inside File Sharing Preferences, you use the hidden "trick" of holding the Control key while you click on That folder, and described in this article:
Backup disks you can use with Time Machine - Apple Support
... AND File Sharing MUST be enabled over SMB, not ONLY Apple File Protocol.
The format of the destination drive does not matter, because a sparse-bundle disk image will be created inside the shared folder.
Once the destination folder is set up on the Mac Pro, you tell Time machine on the iMac to use it as its destination.
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You can alternate your old locally-attached drive with the new Network backup on the Mac Pro until you are confident the new one is working to your satisfaction. Every-other backup automatically goes to every-other drive, and the new destination will be a new stand-alone backup starting with today's data.
when you use time machine APP (not to be confused with Time machine control panel) to look at back at what is available for restoring, you will see the composite from both backups combined into one unified presentation.