Thanks for your reply!

I'm assuming these different combinations outlined in the manual refer to internal storage only.

If that is the case, I can always add more external storage in the future, in case I need it. The files that will surely take up most of the space are 4K videos that I need to store.

@nvInterstellar , you are correct.  The storage configurations are for internal setup.  Depending on your purposes and usages, external storages could be added to your needs.  

You didn't mention about which graphics you are using.  So if you are not using dedicated graphics, you should have free PCIe slots, which can be used to add more storage.  With proper adapter, you can advantage of the high performance lanes to add more NVME drives.

The external storages can be added with USB drives, NAS drives.  It's a good option because the external storages will be accessible for multi machines, users.

@Chino de Oro , I'm using the Intel dedicated graphics that is shipped.

But if I add more NVMe drives and exceed the maximum storage, won't this represent a "violation" of the supported configurations?

@nvInterstellar , it should not be a problem if you are taking advantage of unused lanes to expand your storage.  Also it is not economical to add several TB of SSD for media storage, just increase the hard drive capacity.  Although it is a budget model, you got an enterprise class workstation which is capable of running business applications, graphics rendering, intensive video editing, and it will outperform those regular home PC.  If you are going to upgrade to higher capacity hard drive, be careful and try to avoid SMR drives.  I hope you will enjoy your new machine.

Thanks for the detailed advices, @Chino de Oro!

Just put in a thunderbolt card and use external drives, as many as you want.

There's no limitation on external storage.