I could finally resolve this issue. On a similar Devon thread I found that Catalina absolutely requires Full DIsk Access turned on in System Security & Privacy Prefs. I turned it on, and for good measure also tossed my current Apple Mail plug-in and reinstalled it as outlined here:
After restart, all is well again. Image Capture photos from iPhone are displaying normally and Email import is also displaying much faster than before.
Thanks Alan for your attention and support. Thanks to all the Devon staff for keeping good support threads available.