Hi James, some more results and questions!
Since ditching the 512 MB card, logging has been much more reliable.

The time is still wrong, not a big deal, the 1-minute setting logs every 25 seconds or so.

I am seeing small glitches in the recorded temperature. In one log file, I found single sample downward jumps of 1.1 degrees C, 24 hours apart. In another log file, single sample jumps up, 0.11 degrees C, a bit more frequent. It’s too soon to say whether they occur regularly or at random.

There’s no jump in the other measurements at these times.
So it looks like a small bug in the temperature measurement / processing.