Yeah as others have hinted at this is going to be easier with a router/network level feature.
I'm sure there's a way to do this by setting up a special virtual interface, limiting the bandwidth on that interface and then using iptables to redirect the apt traffic to that interface or something to that effect but I'm not a network engineer and I don't have the experience to help with something like that. There might also be a way to do this with cgroups to limit the bandwidth but again that's above my level of expertise.