mastiflover gives great advice here.
thanks for helping this dooger - he spunds like he could use a great owner
It'll take a little while till the dooger settles in.
Glad you are going to the vet next week as that should answer many questions as well.
In terms of the freefeeding versus fixed times.... fixed times may give you more reliable pooping times. The dog should poop soon I would think but sometimes stress/change can delay bowel movements.
In terms of walking on the grass, is it that unsafe for the dog to walk on the shoulder? would reflective strips on you and the dog make it safer?
Good luck and thanks again for adopting!