I've worked with steel rolled-top edging. It's not difficult to bend into a perfect 90, I generally brace a 16" x 24" steel framing square against the inside edge and bend the edging around it until it reaches 90 degrees.

You can get it from a landscape supplier or just go to Home Depot, where it's $5.98 for 8 feet. I like it because it's safer than the steel edging with rubber cap. If you cut the non-rolled-top edging with a lawn mower, the cut edge acts like a razor, cutting pets' paws and people's feet. Get the steel pins for anchoring it. I place the pins about 3 feet apart except when I'm overlapping the edges of two pieces. At those points, I use two pins close together. I insert the edging into the ground so that only 1/4" extends above grade. That way the edging provides a clean edge, but is nearly invisible.

Sorry, I forgot to mention the thickness. It's about 1/16" thick and typically comes in 16- to 14- gauge. I used the Home Depot rolled-top edging to edge the beds in my new landscape last fall, and it works fine. I've also used steel rolled-top edging from a wholesale nursery supplier. The edging from the nursery supplier may have been a little stronger, but it didn't matter much.

raw steel - so it's not painted or powdercoated?

The steel yard may be selling decking remnants.

Nope, not painted or powdercoated. May be galvanized.

You can purchase 1/4in x 4in A36 flat plate from most steel suppliers they come in 20' lengths . Most suppliers can cut the material to your specific lengths for a small fee if you don't have tools to cut yourself . You can use a grinder with cutting disc or bandsaw to cut through the material . Rebar can be used as stakes but to keep it looking sleek and not have a bunch of ugly rebar showing it's best to weld the rebar to the edging as close to the bottom as possible . Be sure to lay the edging out first where you want it , stake it in place 1st with rebar and hammer it in the ground where needed then go back and weld in the permanent rebar on one side only . After you have everything welded in place remove the temp. stakes . Another tip is to do a string layout to follow so you can keep a perfect straight line. Don't forget to check your levels side to side constantly to ensure your edging looks truly perfect . If you have corners to go around that requires 90° angles use a cutting disc and score the edging about halfway but don't cut through then bend the steel without breaking to your angle . It will give the appearance of a continuous piece of steel that was bent with a break press . Try to incorporate step downs in places where the elevation drops to save money on having to buy more expensive wider steel plate . For rounded curves purchase thinner gauged plate such as 11gauge that you can bend by hand as you go . These are all tips I learned the hard way hope it helps even though I'm 5years late to the party 🥳🎉