The cameras produce different results because you're using the same raw processing settings for cameras that have different sensors and processing pipelines. You need to tweak the settings to match the camera.

To improve highlight detail:

• Don't increase the exposure setting so much. Increase shadow and highlight recovery. This should work if the detail you need is contained in the raw file.

• Reduce exposure when capturing the image. Then use shadow and highlight recovery when processing the raw. This is basically how dynamic range expansion works in some cameras.

Probably the sensor in the D850 is more sensitive, so it looks like it blows out sooner. But shadow detail is probably slightly better. (Sensitivity ≠ Dynamic Range.)

Color correction profiles for different cameras can also have subtle effects that change how settings affect the appearance of images. However, it would not alone explain the large difference in highlight detail.

If the difference between the cameras is consistent across multiple exposure tests, external causes, such as aperture variability and lighting changes are unlikely to cause the differences. (External variability would not consistently affect one camera over the other.) Flickering from AC power cycles is ruled out by the 30 second exposure.