Hi Jason,
The initial inventory get populated by triggering a AWS Config Snapshot. When you add a Config input, the snapshot will be triggered automatically, unless your IAM user don't have such permission.
Setting up config service for all regions and all 9 accounts really is a lot of work. So, let me share some good news (I am the dev manager of this app at Splunk, BTW): 1) in the upcoming 4.1 AWS app which will be available in 1 or 2 months, we will be using AWS REST API to list all resources, so you don't have to configure the Config, unless you want to use the topology tool. 2)AWS itself is enhancing the AWS config service so that you can use 1 region to capture all inventory in all regions. 3) recently AWS config has published a feature to let you capture the IAM user/group/role/permission. We are in the process of enhancing topology tool to visualize those IAM resources.
For the time being, you may have to create 9*9 SNS topics, 9*9 SQS queues. You may use CloudFormation or similar tool to automate it. Or you can wait for 1 or 2 months for the next release of AWS app, but with 4.0 app you can still monitor other data sources, such as CloudWatch, billing, VPC flow log.
Hope it helps
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