> Could you split up the traffic across dozens or hundreds of IPv6 source addresses?

Yes

> I can see how this significantly increases complexity for tracking

Not really. You just track at some prefix level. In general, the ISP will hand out a /64 per consumer so that's what you can track. From there, you can build more complex and more precise grouping rules for tracking.

I'd mix in some IPv4 of course, maybe pipe some of the connection via VPN interface so the physical route is not same for all packets.