Given the steps that you have already tried, the following further steps may help:
If you have ever used the Workout app and/or any third-party fitness, heart rate or sleep tracking apps on your Apple Watch, check on your watch to see whether there is an active or paused activity within any of them.
- If there is, end that activity from within the app on your Apple Watch (not your iPhone).
- If in doubt, remove any such third-party apps from your Apple Watch.
Activity-related estimations depend, in part, on your personal information. To check that this is accurate - and to update it over time:
- On your iPhone, in the Watch app, go to: My Watch (tab) > Health > Edit - tap and adjust items, then tap on Done.
Calibrate your watch, being sure to complete some calibration workouts (which teaches your Apple Watch how your arm movements relate to your stride lengths when walking or running at different speeds) in addition to resetting existing calibration data (which I know you have previously done).
Keep in mind that you can easily accumulate a significant number of actual steps taken when moving around within your home, without going out. However, you obviously should not be receiving credit for steps that you have not actually taken.
If the issue persists, then I suggest that you contact Apple Support (remote diagnostics and/or mail-in service may be available, if required), make a Genius Bar reservation or visit an Apple Authorised Service Provider for assistance: