Thank you to @
atwl77 and especially @
njkayaker for the very helpful advice.
So, I did a test ride this morning into work with the basemap disabled (this is what I meant by "turning mapping off"), turn guidance OFF, and course points ON. Major improvement. Course Point distance is fine and works as expected.
However, I've run into the problem described in
this thread on Garmin forums: "Location at Next" [Next Point] is a blank field. This is infuriating and nearly a deal-breaker for me. This used to work fine in 500s by reading the next line in the .tcx file, but according to this thread,
"It seems that the data field Location at Next interacts with base maps (which doesn't seem to make much sense)."
Very annoying. My last hope is to re-enable the basemap, and perhaps something will 'click' such that I will get Location at Next? I'll have to wait until the commute home to test this.