Sorry for the late answer here.
As a deployer, I would prefer using the "official placement" build processes and artifacts.
1) I like the idea of building images using openstack-infra zuul pipelines inside placement project scope.
2) I believe the helm charts should be discussed with the openstack-helm team, as they can probably share the best practices and/or can carry this for you (with your help).
I would be happy to discuss this with you during PTG.