I killed it. I suspect as part of the delidding process it’s now dead.
The sensor always returns 99.90pCi/L when I query it and the official board just displays “Err.” for a reading (same as it shows when over-range). Sanity checking with my other sensor shows that radon levels were not over range, it’s broken.
Likely the sensor itself was mechanically stressed as I cut the solder joints holding the metal lid on. Now to decide whether to buy another RD200 meter or add a different product to the ecosystem.