1. Make the j-pole out of copper. And solder it together, like a plumber would.
I can't see why you would want to make it out of steel. Like others have said, once it starts rusting, the threaded connections are going to blow the swr out the water.
2. The larger the diameter of the pipe, the greater the bandwidth with a j-pole. But the larger the diameter, the greater the weight of the thing.
3. I get tickled when people say it's not a good antenna. When I had one up, I was full quieting into repeaters 75 miles away any time of the day or night. I used to work a repeater 160 miles away most mornings.(by noon it would fade out).
I'd have to see that for myself. Any other details, how high up, how much power, what is between your QTH and the repeaters. Were you on top of a mountain when doing this?
I know what I can work with a high gain antenna, set up with an antenna analyzer, almost perfect SWR in the range of several repeaters and running 80 watts from a Yaesu FT-2900, short coax, LMR 400.
No calling you a lier, but as I said, I'd have to see that for myself and have more information. J poles do work, but are in the negative gain zone by the time you run any length of coax to one. I also have made and used J Poles and never had anywhere near that kind of range from one. Good for local use or back up, but long range, you can keep them.
For soldering J Poles, you are supposed to use solder made for electronics, not plummers solder, two different animals.
73's John