What I have done with my 1TB external backup drive is to add one additional APFS volume with a reserve size of 400GB that I name "Files" so that I can use the same external drive for both Time Machine and general file storage.
Here are the steps that I used with a new, empty Samsung T5 1TB SSD:
- Connect the Samsung T5 drive to the Mac and open Disk Utility.
- Right-click the Samsung T5 drive in the left-hand pane and select "Erase".
- In the Erase dialog, set Name to "TimeMachine", set Scheme to "Guid Partition Map" and Format to "APFS".
- Right-click the TimeMachine volume that was created above and select "Add APFS Volume".
- Name the new volume "Files" and click the "Size Options" button.
- Set the Reserve Size to 400GB (or however much you’d like to reserve) and click OK.
Once you have done this, you can open the Time Machine utility and select the "TimeMachine" volume as a backup disk, and still have a separate "Files" volume that you can use for general file storage using typical utilities such as Finder.
These steps don't necessarily have to be done before configuring Time Machine to use the external drive. I was able to add an additional APFS volume to a drive that only has one volume that had already been set up for use with Time Machine by right-clicking the existing volume in Disk Utility and adding a new one.