Ducato on stony gravel corrugated roads.

Fit to what Dennis?

Dennis.
It seems you are pretty well locked to the AlKo struts on the front if you want to change that end. Perhaps ask AlKo how their offering performs and how it is superior to the Fiat strut. They have to be able to articulate a sensible account of why it is better. Hopefully their strut shock action is superior because that is essential. The spring basically just holds it up.
At the rear you also need a non fading higher performance shock. With the springing or assistance of, you need to have suspension travel upwards to absorb movement UPWARDS. If you use Sumo or airbags which usually have not much up travel and limited bump absorbing ability, you aren't gaining much other than load carrying ability when they are flattened or pumped up harder. For a suspension which absorbs and has a ride quality, it needs compliance and shocker control of the mass involved.
Therefore, I prefer an additional spring leaf, adjustable if possible AND good quality shocks. Not usually ones recommended by 4wd stores who are not suspension people at all, they just sell the stuff. Do research!

Dennis.
The tyres are the primary suspension and so some profiles, ie, higher profile may also be an advantage. Also ensure they are able to withstand sharp stones if the stoney is some outback roads.