Audit Client API endpoints

[dmallory42]

Description

We recently had an issue (#10713) where we were using a GET request instead of a POST with an extremely long query body.

This issue covers auditing the codebase to make sure we are using best practices across our internal API calls to the client API.

Acceptance criteria

• We are using the appropriate GET/POST/etc.
• Our route definitions (e.g. validation/arguments passed) are accurate and follow good practices.