Gas line for new stove going from T to termination plate
Home Depot lists CSST ½" pipe at 25' here.

Lowe's has a lot shorter flex pipe here.

I don't know why you couldn't put the flex through the floor and then have your shut off valve.

I don't know what you mean by an "excess flow valve" but I would just pipe it straight through with the shut off valve in there somewhere.