A high tensile steel(drill core tube)is use for the rear axles on class 5 or for LLM 3/4"square tube telescoped into7/8"square welded to chassis spine'
Steel conduit is not physically strong enough, as it is a thin wall tubing and screw together fittings would also collapse under torsional stresses.
The wheelbarrow wheels are usually fitted onto a 3/4 bolt, which is welded(with head removed) to the rear axle or front forks. We use an internal spacer fitted over the bolt so that when the self locking nut(nyloc) is tightened it does not put compression stress on the bearings inner races.

all my steering rods are electrical conduitsgreat telescoping sizes. but they are only availabe up to32mm Od here after that they call it by other names and dont sell it with a threaded end. for example 2" or 50mm makes a great rear axle for a class 5 as long as the Y frame is not to extreme.the 60mm OD stuff we use for our mini chassis and class 5 mast steps is easily able to take the loads. its brilliant stuff for building landyachts.
starting to experiment with these more readily available materials led to the explosion of minis and new designs as it is a material that is more readily available in common sizes . to join a axle to a stubb axle consider welding a piece of flat plate ,say 10mm thick to the axle end and then weling the stub straight on