Easy place to see platypus is the lake in the middle of Penguin on the north coast. Or the salmon ponds at New Norfolk. We may have been lucky but both spotted in the middle of the day (and two platypi(?) in the lake in Penguin).
We revisited Tassie in November and three times in two weeks had to slam on the anchors to avoid echidnas crossing the road.
My top tip for nocturnal animals is Bruny Island. We try not to drive at night to avoid the carnage but got caught out. I very nearly got out of the car to shoo away suicidal local furry things (including quolls) on the road to Barnes Bay after you turn off the road from the ferry. Very stressful and slow journey but we ticked off almost everything there was to see.
Lots of wombats in the fields north of Port Sorell and on Maria Island.
Not seen a devil in the wild though.