Hi Peg!!
I've been watching your build progress here and on another board.
It bring's back memories of back-breaking work four years ago. But I had a back-hoe dig my main 'pond hole' but the final digging and plant shelf dig in rock and clay was not easy.
To your question re great stuff. Sure use it, and as mentioned, it can be trimmed.
To avoid the unsightly yellow/gold color, before you add your water, you can paint it OR use 100% silicone and glue stones/rocks to cover the expanded foam. I used both GS and silicone to glue flat rocks to part of my liner too.
Good luck, and ENJOY,
fred