I once tried to use galvanized flashing on the uphill side of a rock retaining wall. It was much wider than any garden edgings which tend to be less than 6". Unfortunately it substantially degraded over half a dozen years. My soil pH is around 7.2, but it is a calcareous clay. Calcium and chlorides (salts) cause corrosion on galvanized metal and are often found in both city and well water. Chemical fertilizers are another source of chlorides. The farther away from neutral pH your soil is the worse the corrosion will be. How wet your soil stays also affects corrosion, so having a free-draining soil may help.

Hot-dip galvanized offers the best protection against corrosion. If you use metal roofing you should install wooden posts to screw the metal to. And, use the fewest number of screws possible, because each hole is where corrosion will begin. Metal roofing comes in lengths starting around 8' long, but they are about 26" wide, so you either bury the extra width or you have to cut the pieces in half.

The reason people use the heavy gauge steel is because it is thick. Once it rusts the rust acts as a barrier to further rust. I suggest you look up a metals supplier in your area. Do a search for "where to buy sheet metal near me". You need a place that will sell you the metal, but if you have no experience with bending or welding you will be better off hiring someone to install this. Whatever materials you use you will need tools, some of which are expensive. And, there is no substitute for experience. I suggest you do a search for videos on how to install whatever type of retaining wall you want.

After seeing the picture, I don't know how you think this looks good. It looks like a way to seriously devalue one's property ... what new owners couldn't wait to get rid of if the house ever changed hands. And what a job it would be!

Of course how one uses their space is up to them because that's how private property works. But so often, personal opinion is going to boil down to good taste vs. bad taste. And while "not everyone" will consider it a devaluation of property, the vast majority will (we're talking about what is shown in the picture) because most people have no need for such a garden, with its mountain of imported earth occupying such a large and central portion of their backyard. I would bet anything that if we could see the site, we could think of a much better and less offensive way to accomplish the overall goals.

The main things that bothers me in the included picture is how narrow the path is (difficult to use a wheelbarrow or wagon in that path!) and the excessively curvy lines (which would contribute to the difficulty of moving working gear along that path!) So, whatever border material you end up with, keep the lines simple and functional with ease of movement for yourself and maintenance equipment in mind.

Yardvoork

Thanks for your opinion. I am planning this design on the sideyard, wasted space uphill now.
It is not front, or backyard.