How to defend against flat smashes /drives to backhand

Sounds like your racquet might usually be below your waist and also pointing downwards to the floor too much.

You can try defending with the racquet shaft a bit more horizontally. This would mean less distance for the racquet head to move up for flat smashes and drives and you would still be able to defend steep smashes. That’s quite a hard habit to change though. Another solution to help get the racquet held more horizontally would be to emphasise defending using a crouch position – many casual players will defend in an upright position.