I've not tried bending tube yet. As stated, new to and feeling my way around metal so this is all extremely helpful. I'd focused on stainless for the base shine as it seems the color of many coatings are affected by the underlying reflection or lack there of. My thought process was, if I can find polished tube to start with there may be less work coating and plating the section I form rather than coating the entire piece. Also for its rust resistance because these will see all seasons. I'll switch gears and start looking at carbon steel unless you can suggest a more polished option.
I'd also been scoping something like this to bend with. Would you suspect your suggested anti-crushing techniques are necessary with these presses as well or was it strictly a matter of changing my proposed material?
Bending the tubing is going to damage the surface. You will likely have scratches and maybe even gouges depending on your tooling. I wouldn't worry too much about surface finish of the raw tube, you're going to end up polishing it anyway. Even if you buy 300 series stainless it won't be polished when you get it, unless you pay extra to buy it in polished form. Stainless isn't naturally shiny, it needs to be polished.
Crushing happens because the metal has to go somewhere when it bends. When you bend a tube, the centerline doesn't change length, but the inside and outside surfaces do. Lets's say you're bending a 2" tube on a 4" radius. The 4" is the dimension of the bend at the centerline of the tube. If I'm looking at it from the top and bend it to the right, the right side of the tube will be on a 3" radius, the left side will be a 5" radius. In order for the tube to bend, the outer edge has to stretch, and the inside edge has to compress if you want it to stay round. If you leave it to it's own devices, you get the outer surface going flat instead of round and the inner dimpling and wrinkling. Or the outer will tear and the inner will just kink, or any number of other things.
The deeper the bend, the more distortion you're going to have. It doesn't matter what tool or material you use, the material has to stretch and compress. You've probably seen the term "mandrel bends" or something similar. The mandrel is a piece of tooling that goes inside the tube to hold it's shape as it's bent. If there is nothing serving as a mandrel, you're going to get crushing/collapsing/folds/dimples as the metal stretches and compresses. And with a mandrel you can get tears when the material won't stretch like you want.
Material makes a difference but you're going to get distrotion no matter what material you use. Anything with walls thick enough to not crush won't be something you want to bend at home.
The tooling you show will crush tubing just like every other non-mandrel bender. You also have to be able to buy specific dies for the specific size tubing you plan to use. FYI tubing and pipe benders use different tooling because they call out sizes differently. One uses the ID for the stated size, the other uses the OD for the stated size. The tooling has to fit the OD of your tube. Close will not be close enough. If you are using 20mm OD tubing you need 20mm dies for your bender. If you change to 0.75" OD tubing, you will need 0.75" tooling. I'm guessing imperial sizes will be easier to find in the US than metric, but it's been a long time since I've looked for it in either units.
Another issue with the tool you posted is it has to be narrow enough that you can bend the tubing 180 degrees and still clear the tool, as in it needs to be narrower than the space between the finished sissy bar.
FWIW - I worked in the automotive exhaust industry for about 30 years. Bending stuff was never my job but I've had to deal with it quite a bit over the years. Have you ever seen exhaust pipes with what looks like a corrugated surface on the inside of the bends? That's another way to use up the excess material when it bends. It's gotta go somewhere.
Honestly, if I was going to bend up a sissy bar and only planned on making one of them, I'd use solid rod. Not kidding. Heat it up and bend it around whatever you have around that's the right diameter. OR... fill the tubing with sand and treat it like a solid bar. Forget all the tooling and just use heat and brute force.
EDIT: If you go the sand filled route, you'll likely have to fill it up and cap the ends so the sand doesn't move when you bend the tube. If you do this, leave a vent hole somewhere. Moisture in the sand can turn to steam when you heat everything up and steam = pressure, you don't want a pressurized tube to burst/tear and shoot hot sand everywhere.