The way to express the design in a pure-IPv6 world would be that you use ULA addresses to reach the AWS services that you use and globally-routable addresses to reach the outside world.

Given that the cost that we're avoiding paying with the mechanism I described in my previous post is the ongoing cost for globally-routable IPv4 addresses, I'm not sure what cost you're talking about paying.

And given that the benefits are not having to pay for globally-routable IPs, I'm not sure what benefits you're talking about that we don't get?

Are you perhaps one of those "Hosts must be IPv6-only, no dual-stack allowed!" people? If so, I regard that as a silly stance today, and expect it will remain a silly stance for the next several decades (maybe even the next century, who knows?).