How can I configure an AWS IAM Identity Center for an organization of small user base?

The IAM Identity Center documentation walks through the setup, which you can find here. The documentation also provides guidance for how to assign permissions, how to manage users/groups, and how to use multiple accounts. Like you said, the setup can be simple but if you follow the docs and take into consideration the points within the guide it will be appropriately set up.

In terms of secure, do you mean the security of the IAM Identity Center service or the permissions you are granting to you new developer account? If it's the former, please see the Security documentation for IAM Identity Center here. If it's regarding the assigned permissions, I would encourage you to utilize the least privilege principle and only grant permission to what the developers need access to.

Here are some other links to documentation that may be useful: Security best practices in IAM Techniques for writing least privilege IAM policies