From what I’ve read, pure iron oxide is not attracted by a magnet, but what comes out of our tank may be other stuff that magnets do attract, and the iron oxide clings to that other stuff. At some point there may be tiny flakes of iron oxide that are not clinging to another material so will bypass the magnet.
People on this forum have shown a “dead leg” to help more particles settle out of the fuel before going to the carb. It’s basically a T fitting interrupting the horizontal fuel line, with a tall (several inches) pipe descending vertically. The original cast iron sediment bowl (not the glass kind) is not very deep. I am guessing the dead leg would work even better if the fuel flow velocity across the top is slower, giving the particles more time to settle out. This would suggest the cross section of the horizontal line should be as large as is practical.
(I searched online for “dead leg” and got a variety of other meanings, so maybe this is not a typical usage of the term.)