Increase Rise and Fall Time of this Schematic
Add gate-drain capacitors on M3 and M7.  You can also add a series gate-drain resistance, but you already have effectively 500 ohms on each.  1000pF should get you in the range of 2us, but you could probably dial it in more experimentally.  If you want to use a smaller cap, you could add series gate resistors.  This is one of the better controlled methods, because the feedback is right on the output stage where you're trying to create the effect.