Just because you know objective-c doesn’t mean you know a damn thing about raspberry pis, backup programs, NASes, or anything else. It doesn’t mean you know or want to manage your own network infrastructure. They’re a Mac app programmer, not a Linux professional, not a micro-computer professional, not a network engineer, not a sys admin.
Time Machine wouldn’t work here, because it needs the files locally and he’s already stated he doesn’t have a 6tb drive.
> he’s already stated he doesn’t have a 6tb drive
Someone who uses a $500 gift card to renew subscriptions could afford one
Which is not an unreasonable thing at all considering it's literally marketed as a storage solution for your photos, and a top of that even encourages users to store originals only in the cloud.
A simple usb hard drive will actually do, no need for a NAS. The only action required to implement proposed solution is to check "Keep all data on this Mac" in both photos and iCloud Drive settings. And to be extra cautious add a second backup drive from another vendor (to be extra extra cautious don't use Time Machine for the second drive).
For the specific case of thoses that don't have a big enough internal drive they might need to store data on an external drive. But if you do have 6TB of pictures you normally should ask yourself if a RAID1 or RAID6 is not warranted at this stage.
In conclusion it's not a binary decision there is lot of room between "I solely rely on the cloud" and "never trust the cloud".
Uh, the guy writes programming books for a living.
But since he's all-in Apple he could just use Time Machine to some sort of NAS and get a more streamlined version of the above.