Ultraviolet sensor response curve
After much research, I purchased a calibrated 300 nm-calibrated sensor which did what I wanted. The cost was 3x the cost of the Extech meter (about 800 EUR) which is reasonable. Interestingly, the unit came with a motorola smartphone which communicated with the sensor via an app. My LED is rated at 30 mW, and my power reading was about 12 mW/cm2 from a re-focused spot from the LED which went through two UV lenses. Since the entire spot fit on the sensor (you could even see the blue spot by eye), I presume this means that the output is abt 12 mW.

https://sglux.de/en/datasheets-sensor-probes/uv-surface/