Never bent square tube yet, but do a lot of round, is the bender setup for a mandrel or not? If mandrel bending you can go a lot tighter as your very much controlling the distortion. If the weld seam opens then its better to find a different steel supplier, because its fundamentally flawed tube. Good tube and the weld seam should not be a issue till you approach the fail point of the parent metal.
What makes tube bending hard is theres a lot of variables at play at once, tube od, wall thickness and center line radius, theres various calcs for round tube that let you kinda predict how it will go and how much you need to support it, as to weather you need a wiper, mandrel or can simply just bend it with a follow block and skip thoes extra parts! im sure the same applies to square or rectangular material too, just probably with a different correction factor.
Probably supported in a die and follower the material should never crush, just more a question of kink comes into play and what your preprepared to accept distortion wise, if it was round tube, at that wall you would be wanting to use a manderal for internal support at more or less any radius, this is the best place i know for this stuff and there calcs hold true in my experience, all be it thats all sub 1" erw sub 14G wall thickness’s. Not sure what they have on square bending though!