5 minutes is way too much for blitz with only 10 second overtime periods. Such a stark difference encourages meta-gaming in order to have a time advantage over the opponent. Main time should mostly be used to compensate for the time taken by greetings and first moves and such at the start of the game. If you want to add lenience, a much fairer way would be to increase the number of overtime periods. Those are also far more likely to be useful in the late middlegame fighting.