I was surprised IPv6 was only briefly mentioned! Is that something you're looking at next, or are you already running an IPv6 egress network?
Of course not every destination is an IPv6 host, so IPv4 remains necessary, but at least IPv6 can avoid the need for port slicing, since you can encode the same bucketing information in the IP address itself.
I've seen this idea used as a cool trick [0] to implement a SOCKS proxy that randomizes outbound IPv6 address to be within a publicly routed prefix for the host (commonly a /64).
I guess as long as you need to support IPv4, then port slicing is a requirement and IPv6 won't confer much benefit. (Maybe it could help alleviate port exhaustion if IPv6 addresses can use dynamic ports from any slice?)
Either way, thanks for the blog post, I enjoyed it!