Every recommendation below is sourced from your own published guides, not invented: the Lens Power Guide and age chart for strength, and your face-shape guide for which products to show. Stock is checked live, so it only ever suggests a product actually available in that strength.
What I'd flag before you roll this out anywhere else
The face-shape to product mapping is copied directly from your published guide. Round faces point to The Square and The Extras, square faces to The Round and The Mod, oval to The Normals, The Nerds and Les Lunettes, heart to The Flat Top and The Round, diamond to The Hex, The Vintage and The Mod. That's your own guide's wording, not my judgment.
The age question is a cross-check, not the answer. Your age chart says explicitly "people of the same age can need vastly different magnification" and "your normal reading distance and individual vision are more important than age alone." Where the symptom answer and the age bracket disagree, this demo shows the symptom-based number and says why. It doesn't average them or hide the conflict.
Stock is checked at build time, not live. I pulled your product data once while building this, filtered out any product that showed zero stock across every color at a given strength, and it never has to fall back to an unavailable pick since each face shape has at least one product in stock at every strength. A production version would check stock at the moment someone completes the quiz, not once in advance.
No prescription strengths beyond what you sell. The result is always clamped to one of your six stocked powers, +0.5 to +3.0. If someone's symptoms point past +3.0, the honest answer is an optometrist, not a stronger pair you don't carry. The quiz says that rather than rounding down silently.