My simple approach: keep a copy of the app as a test version and try changes there first. Is there a better way for solo builders?

A test copy is the right idea. What helped me on top of that: point the test copy at a separate test database or sheet so you never touch real data, keep a short checklist of the 3 to 5 flows that must always work, and take a snapshot or version before every change so rolling back takes seconds.

Using a separate test database or sheet is a good safeguard; I like pairing that with a small must-pass flow checklist and a version to roll back to.

That combo has saved me more than once. One small extra: write down what you changed and when, even just a line in a note. When something breaks a week later, it's much faster to trace back.

A lightweight change note is underrated; pairing it with a dated version makes rollback and debugging much less mysterious. For a solo workflow, I’d keep the checklist to the few paths that would actually block a user.