One kitchen place I went to suggested putting the gas tank outside the kitchen, instead of inside a kitchen cabinet, for safety. But in my kitchen, the stove will not be located at the back wall, it will be at the side wall so the hose would have to travel probably about 2m behind the cabinets then through the wall to outside. Will this affect the stove efficiency? I think if I need to change the hose later for any reason, it will be a big hassle....

Has anyone else done this?

Actually copper are not a good idea for gas lines.
See how LPG destroy your burner head can know already

I still prefer using rubber hose.
Mine already last for 8.5 years and never leak.
The best way to make the hose last long is depressurize it and remove the head from cylinder.
LPG have some impurities inside, so it may corrode the metal piping over the time.