Actually, AWS does now support building AMIs and sharing it across an AWS Organization or OU: https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/share-am.... If you're using Packer you want ami_org_arns to share with an org or ami_ou_arns to share with OUs https://www.packer.io/plugins/builders/amazon/ebs#ami_org_ar....
You do still need to create separate AMIs per region though