As I have said, you don’t have to have a fixed axle, and there are some advantages, too, such as being able to drop the axle boxes, wheels, axles and gearbox as single units without disturbing the quartering: useful for maintenance and painting, and also smoother running as the body does not move in accordance with the fixed axle. It can be slightly more fiddly to get the first axle square, but a simple jig made from a block of wood with axle-diameter holes drilled at precisely the correct centres, with axle material inserted, can help here if you don’t wish to buy one of these chassis assembly jigs.
Hate to say it, but I did recommend this path.
Having a fixed axle is useful as a reference for setting the other other axles using the coupling rods as a jig, but only - and only if - the fixed axle is dead square to the longitudinal axis of the frames.
Mike Sharman recommended fixed axles in his book on flexichas, but did demonstrate how to have a moving axle as well, and as he said, the book was subtitled as a way to build chassis, not the way. Iain Rice popularised the fixed axle, along with (I believe) Rod Nero’s original Perseverance Kits.