Is there a way to collect hrv value more frequently ( for every 5 seconds or so) from Apple Watch?
Heart Rate Variability
No, there's no way for third party apps to trigger HRV readings from Apple Watch (unlike heart rate which is possible).
However, since the new AF History feature launched with WatchOS 9, I've noted that enabling that changes the number of daily HRV reading the watch records from roughly 5 to roughly 50. Users have to enable this feature though and it currently "requires" the user to have a diagnosis of AF. Hopefully the additional HRV readings roll out to all users with WatchOS 10.