@Chriisii21 Regarding heart rate, and having also a S9 baro I do not have face problems using a Polar H10 strap or Suunto Smart Sensor. Are you sure that is tight enough or wet? Could be that when you are in the MTB it moves and loses contact? Is the battery new?
Regarding the fitness level, in my case is quite reliable but a few hints: Define correctly max heart rate and rest heart reate. As far as I know it only works with “running” activities (running, trail running, etc…). Besides if your heart rate is not measured correctly it will give bad/false values.