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.