I couldn't agree more; client certs are too much work and come at a non-trivial maintenance cost, same goes for AD/LDAP. So JackDB also acts as authentication provisioning front-end for a DBMS so users don't have to do that themselves and suffer the maintenance cost/overhead. Interesting.
I'm curious, your VPC bastion; how do you make managing SSH keys scalable given that setup? And why not separate accounts which in fact help with keeping sane logs of who did what, and distribute the risk of compromise across several accounts?