Headers, dependency hygiene and backups you have actually restored.
The short version
Most problems here are not technical. They come from decisions made without a clear constraint. Write the constraint down first, then pick the tool.
We measure before and after every change, keep the diff small, and ship behind a flag when the blast radius is unclear.
Measure first. Opinions are cheap; traces are not.
What we do on client projects
Every engagement starts with an audit and a one-page plan. The plan lists the three changes with the highest return, an owner for each and how we will know it worked.
If you want the same review for your product, get in touch.