Is a lens protector recommended, or does it interfere with camera operations?

Recommended? Depends on how much risky usage you have with your iPhone.

My Lifeproof case protects the entire body of my iPhone SE, including a covering over the camera. Does it at least partially block the natural light into the lens? Probably a bit. Enough to bother me? Not really, for now, since the iPhone is safer.

For you *any* cover that even partially filtered light to the camera lens may be too much. The only thing you can really do is try a cover and find out.