Hoo_Dat Does anyone have any suggestions as to how best to troubleshoot this issue?
There is nothing to troubleshoot, it is the nature of the beast.
Remember that the Radian Quad has four bands, each with about 5 nm bandwidth, so a total of about 20 or so nm.
A camera with a UV/IR protect filter will pass between 450nm to 750nm or so. Wideband sources like most stars, will deliver 300/20 more photons to your camera (i.e., a factor of 15). Modulo the curve of the camera sensitivity.
With the Radian Quad, you need to improve your exposure factor by more or less 15 to get the same number of photons to reach your camera.
You can get this exposure factor with a combination of exposure time and gain.
If you keep the same gain, exposure time would need to increase by 15x.
The ZWO cameras specs use a gain factor of 10 per decibel (dB). So a gain change from 0 to 100 corresponds to 10 decibels difference, or an exposure factor of the square root of 10, or about 3.2. A ZWO "gain" change of 200 would correspond to 20 dB, thus an exposure factor of the square root of 100, or an exposure factor of 10, etc.
With a gain change of 100 (ZWO units), you would need to increase exposure time only by 4x or 5x.
If all else is hopeless, what you can do in the future is try to calibrate your focus using a very bright star and a Bahtinov mask, with and without the filter. Record the number of EAF steps between them. You can then remove the filter from the optical path, focus without filter, and then reinsert the filter in the optical path, and manually make the EAF step adjustment. Remember to take into account the filter backlash.
If you can't even do that, measure your EAF travel. Then assume that the Radian uses a glass with refraction index of 1.5 (I have not measured my Radian's refraction index, but it is probably close to 1.5). The 3mm thick filter will then be equivalent of a 1mm extra optical path. Next, measure your drawtube distance. Now move the EAF step by about 1000 to 10000 steps and measure the drawtube dstance again. This will give you how much to move the EAF to get a 1mm move to compensate for a 3mm thick filter glass.