Despite the Garmin Venu handing the Fitbit Ionic its ass in my first test of exercise tracing, the Fitbit didn't fail completely in sleep tracking.
Based on my self-perception of how well I slept, including my (and Parker's) acute awareness of the squall line that pushed through around 6:30, I think the Fitbit might have recorded my awake time more accurately. The Garmin, however, also recorded pulse oxygen, respiration, and can display movement on the UI.
Here's the Fitbit results:
Also, yesterday my Fitbit counted 18,206 steps to the Garmin's 12,142—but I put the Garmin on my wrist at 1pm. Adding the 6,963 the Fitbit recorded before 1pm, that means the Garmin and Fitbit differed by (19,105 – 18,206 = ) 899, or 4.7% for the 11 hours between 1pm and midnight. Today I'm using both of them, though I switched wrists (yesterday the Garmin was on my dominant side; today it's not).
The thunderstorms from earlier have pushed on through the area, so once I digest my omelet and my Garmin fully charges, I'll do a longer test of exercise tracking.