Writing down the first version before looking at any starter kit is the step that makes the rest of the audit possible. Without it, every feature in the kit looks useful, and you end up maintaining code for problems you do not have.

Tracing auth as a dependency chain is a good way to put it. Login is rarely the problem. The problem is everything that assumes a particular session shape, from middleware to billing to admin screens, and changing it later touches all of them.

The billing lifecycle checks are where I would spend the most time: renewal, failed payment and duplicate webhooks. A kit that handles the happy checkout and nothing else is a demo, not a foundation.

The keep, remove and replace table with a go or no go at the end is the same discipline we use when scoping fixed price builds. It forces the decision into writing, which is where most of the hidden cost becomes visible.