Massive pk-pk ripple on output using isolated DC-DC converter module
It's been some time since I opened this question but I wanted to share the ultimate solution to help others new to diagnosing switching mode power supply characteristics (many of the comments helped but @VoltageSpike and @Elliot Alderson really set me on the right path).

In the end this was mostly a matter of not understanding how to probe the transient voltages produced by an SMPS properly with an oscilloscope.  After reviewing several different references:


EEVBlog bypass capacitor overview: https://www.youtube.com/watch?v=1xicZF9glH0
EEVBlog grounding strategies for single ended oscilloscope probes:  https://www.youtube.com/watch?v=xaELqAo4kkQ&amp;t=821s
EEVBlog measuring power supply ripple and noise: https://www.youtube.com/watch?v=Edel3eduRj4
CUI's guide on measuring ripple and noise with single ended probes: https://www.cui.com/blog/how-to-measure-ripple-and-transient-in-power-supplies


Using a "paper clip" ground to create the smallest possible measurement loop made a dramatic difference in the quality of my measurements! (as shown below)



Here is an example of the ripple and voltage transients which originally motivated me to post this question.  In this case the probe tip is applied to the 5V rail and the probe ground is connected to a grounded mounting hole several inches away using the probe's alligator grounding clip (pk-pk ripple is 386 mV):



Now that same supply, with no other change and supplied power by a bench-top power supply, is probed using the "paper clip" ground point directly across the bypass capacitor nearest the output of the DC-DC SMPS 5V output (pk-pk ripple is 20.1 mV, well within the DC-DC module's specification):



In the end, this new probing method even allowed me to pinpoint a previously un-diagnosed noise source in the form of an ornery charge-pump based TTL -> RS232 transceiver which could be squelched by adding a second 1 uF ceramic bypass capacitor near the source.