I use Textra for the same reason. I used Evolve before and I'm sure there is a setting somewhere in notifications to turn off the pop-up notifications. I just can't recall where.
One thing that I like about Textra is that they now fully support the Lollipop 5.0 Priority notifications. So if I turn on Priority for 1 hour, the only time Textra makes a sound for notifications is if that contact is marked as a favorite. All other texts from other contacts are silent. Very nice.