Hi there! The roads look really good.
Wen it comes to having modular roads staying flat, just try mounting them on cloth and building them from DIY acrilycs, sand and paint instead of white glue and sand.
My roads are long, can be folded and come flat in minutes.
Have a look:


They are made from teddy fur and DIY acrilycs, sand and paint.
Cheers
GS