I agree that
Zillertal gives you the most options as you have huge vertical, tree skiing and the best glacier skiing in Europe. Lanersbach would be my choice as it goes up to 2500m and is closer to
Hintertux than the other villages in the valley.
Remenber, most other glaciers in Europe are fairly flat and best skied in April/May
The next best would be
Gasteinertal as you have
Sportgastein at the end of the valley which is great at the end of the season.
Re your choices:
Schladming:
Zauchensee in the same area (Ski Amade) is a safer bet. Remember that anything between 2000 and 2500m in Austria is usually good till early April.
Kitzbuhel is unreliable for snow from March onwards.
Kaprun is a safe bet.
Obertauern I would avoid. It's ok for a day trip as the snow is sure, but the place is soulless and there are no trees. Typical of most resorts located on high passes.