Currently, item level cap at 60 and charactor level cap at 70 (this would change when season 2 arrive), you shouldn't worry about not getting relics at level 50ish, as top end gear are at level 60 for the time being.  I would recommend you use the treasure buff only for tarot missions with difficulty +250 or higher.  Tarot missions have a fate cost of 100 or more, you should try the most challenging difficulty that you can complete, and the exp. gain at completion is huge.

Also I think you should be aware of the warzone, where you can start battles without any fate costs and you can get relics from the completion rewards there for battle 25ish or higher.  It has a power rating requirement though, 1100 for the first stage, 1200 for the second stage, etc.  It is a place for getting gear when you reached level 70.