Not sure why youre looking at so many nodes for a cluster. Most of the time scaling the number of etcd nodes does little to help performance, instead focus on giving the nodes plenty of IOPS.
AWS doesn't guarantee that failures within an AZ are independent of each other, so it's not clear how you would estimate what availability you'd gain with this. Losing everything in an AZ + 1 instance sounds like a very unusual and specific scenario to design for.
An AZ going down doesn’t make all other hardware reliable, and equally a machine going down from a cluster doesn’t mean that all AZs are going to be reliable.
Many products have uptime requirements above what Amazon can provide at the AZ or machine level.