Fenix 5S has started playing up in many ways, 2 years after purchase

My beloved Fenix 5S is giving up.  Back light no longer working (really annoying for training in the dark), GPS intermittent, no longer counting stairs/flights, sending me tonnes of out of date calendar and other notifications... I notice the green flashing lights at the back aren't flashing. 

It's two years almost to the day since I bought it.