Quick update for everyone following this: I ended up building a small open-source tool that closes this gap, the Garmin Intervals Bridge.
It’s self-hosted and runs alongside the official Garmin sync. It fills the daily wellness values that sync doesn’t deliver. The average respiration rate during sleep goes into the native Respiration field and night SpO₂ into SpO2, so you can chart both next to HRV and resting HR. A ready-made “SpO₂ & respiration” chart is in the chart library: search for “Garmin Bridge”. It can also backfill past days/years.
Health Snapshot values are (yet) not included, only the nightly average. Setup and details are in the bridge thread.