you can heuristically block ipv6 prefixes on a big enough attack by blocking a prefix once a probabilistic % of nodes under it are themselves blocked, I think it should work fairly well, as long as attacking traffic has a signature.

consider simple counters "ips with non-malicious traffic" and "ips with malicious traffic" to probabilistically identify the cost/benefit of blocking a prefix.

you do need to be able to support huge block lists, but there isn't the same issue as cgnat where many non-malicious users are definitely getting blocked.