I think a nice solution would be height-adjustable suspension (likely using air springs)
It could also be achieved by using a Flexiride axle
- it's still an ordinary rubber torsion axle
, but the separate trailing arms are clamped onto splined ends on the torsion bars, so they can be rotated to vary the ride height.
Flexiride axles seem to be economically priced for retail purchases, so you might think the trailer manufacturers would offer them as standard, but maybe their OEM pricing isn't so good and maybe most customers would never use a facility that might add $50 to the cost of the trailer.