If you have a lot of the weight down the sides of the back wheel, it will be easier to balance. It needs to be back far enough so it doesn't interfere with peddling. Then if you carry too much weight back there, the front wheel will come off the ground when you are not on the bike. You can carry some things on the front to counterbalance this.

Ensure the wheels are strong enough.

The wider the tires, the more weight they can carry.

If it was me, I would start with a frame that is strong to begin with.

Or is this a reason to practice welding. If it is, I could think of a lot of ideas.