Can you share with me your way for tube bending? For me this process seems to be the most complicated. I'm looking for the tools (not expensive ones) or maybe DIY plans that you have used for custom bicycles.
The "anglegrind the inner curve away" process is called kerfing. One cut allows for a slight bend, two cuts, a little more, etc. The closer the cuts, the smoother the curve.
I make sure to clamp a side face of the square tube on a flat surface so that the tube remains properly aligned as it is curved around. A cutoff disc applied to a closed cut with the tube clamped allows for minor adjustment. A tap on the side of a cut with a hammer when the tube is curved and clamped in place helps to "set" the curve, flattening any unwanted side distortion.
You can use a bit of maths for the cut and shut poor man's bend. Using Pi x D / 360 x arc you can work out the difference between the inner and outer edge of the bend. If your cutting disc is 3mm then divide the difference in mm by 3 to get the number of cuts to make and divide the difference by that number to see how to space those cuts.
When I started building bikes, there was some plumbing work to be done in my house and my old 25 year old cheap gas burner was more or less dead, so I invested in a Rothenberger super fire2 torch and map gas, it's a bit warmer than a normal soldering torch.
I also use the burner to bend tubes for my bike projects and it works ok up to 25mm x 1.6mm, I have done 40mm x 1.6mm , it worked for a light bend, see last picture below, but needed more heat, next time i will borrow a second map gas burner.
The method is to fill your tube with kiln dry sand and cap the ends, then heat the tube as much as you can. Then slowly bend the tube around your jig, moving the heat forward as you bend.
The advantage is that you have full control of the shape of bends. But it take some trials to keep the heat constant.
The bend around the wheel could have been better, first time doing a 180deg bend.
A little late to this thread, but if you're looking for a DIY solution, Vince Gingery's Pipe Bending Machine book might be worth looking at (link here). I haven't built it, but it looks simple enough, particularly for those who are members here. The only downside is that a lathe is needed to make the dies, but there can be some creative ways around that.