What can I use for full scale mock-ups other than less expensive lumber?
Fabric can fill in a plane to visually give a sense of volume and does not take that much space to store.  It may be worth investing in making some grommeted panels that can be reused.  Fabric sources could be cheap bedsheets, painters canvas drop cloths, or tarps.  Create a Tinker-Toy-like set of small dimension lumber pieces and fastening plates or bolts to create the structure to affix the fabric panels to.

Tarps can be walked on and can clearly show patio, walkway, and flowerbed dimensions.

I have stacked cardboard boxes to simulate a large piece of furniture I was considering purchasing to see if I wanted to live with that large object in the room.  Appliance and mattress stores may be sources for large boxes that can be broken down and re-used.