Welcome, Susan.
No plywood.
I strongly recommend you move the drain to (or very near) the center of your shower floor. Then fill the hole with sand or gravel to the bottom of the existing concrete, then fill the rest with concrete to the top of the existing slab.
Pewt something (I like 4" PVC pipe) around the drain pipe when you make the concrete patch so you'll have room to install the Kerdi drain later (remove the pipe or whatever before the patch hardens). You can fill the remaining gap with deck mud when you make your pre-slope.
You have purched and downloaded John Bridge's Kerdi shower eBook for this project? Is good tool. Cheap, too.
My opinion; worth price charged.