HRV, low state for a long time

if the watch is working normally, a reset is not going to influence your metrics. Deleting or correcting activities will not either. For all the Firstbeat related metrics, what the watch sees, it will not forget until the data/related prediction fall off whatever time-window is used.

Whenever there is change in HR Max or LTHR, it is better to reset the zones (whatever system you use).As you saw, they don't update properly. 

To reset zones, on the watch, choose reset zones for all the default and sport zones you might have, select another base for the zones, reset again and then go back to your preferred system.

Your HR Max is the cornerstone of pretty much all the metrics on the watch. You need to wear a chest strap for all activities. Make sure you wet your strap, and monitor the HR data during warm-up because there is still a risk of bad skin contact.

A good DIY HR Max field test, if your doctor lets you, is to run your best 5k ever. Take the peak HR of that and add 5bpm.

After you change the HR Max, do a built LT Treshold Test. The combination of your HR Max data (HR and pace), rest HR and LTHR/ LT Pace are key for the HR/Pace/duration relationship the watch is modeling and using for the VO2 Max, race prediction, DSW targets, performance condition and stamina models.

Don't trust the LT auto-detection in particular if you do high intensity intervals shorter than ~20 mn each, unless you increase progressively the intensity during the workout..

Maintain a balanced training program so that the watch is exposed to a variety of efforts across a variety of durations. If you can, add some maximal efforts in there.

After a few weeks, the models will be realigned.