Right. let's have a look at this.
What car do you drive? If it's any way modern, has independant suspension and comfortable stop using it straight away! Go for a drive in an old series LR or a leaf sprung Toyota. Before attacking the problems of the truck have a look at what you expect from it. It is the most primitive structure of two beam axles with a bunch of flexy metal leaves holding the chassis onto them. With a leaf sprung setup the axles are free to move back and forth, side to side and to rotate against the leaves; as well as going up and down as intended. There is nothing rigid between chassis and axle!
It will wander, the axles will go all over the place when bounced around, that's just the nature of the beast. Find soemone with a 814 van that's not too heavily loaded and give it a try; you'll be horrified; they're HGVs not big cars.
Putting 8 tonnes on such small tyres is pushing the limits a bit, but it just about works.
My memory of irish roads is a phenomenal camber, so the moment you put a tyre on a white line it didn't contribute to keeping you on the road, and the other tyre did all the work. You have a minor "bump steer" issue with the offset change, but even that is something one can learn to live with. The slow front end is completely normal for tall tyres and leaf springs. you point the front wheel off the current direction and it has to flex before it can pull the axle to that side. The springs have to bend towards that side before they can start pulling the chassis over in the desired direction. As there's a lot of weight on the front (IIRC you're quite near the max) it all has to preload all the wobbly bits in the desired direction by a fair bit; the very act of preloading the springs actually changes how much the steering track rod has turned the hub on the kingpins.
So it's all a very approximate affair up front. Wheel/tyre combos will change things: less sidewall height and more steel is generally better, but even then you have to be sure of the wheels, and a floppy steel wheel is a death trap.
The rear is more concerning. At 3400kg you're a long way from max loading so the above mentioned preloading of the wobbly bits is much less. Your first phenomenon is the tail working the front wobbly bits. You initiate a turn, the font preloads and starts pulling it all round; then the inertia of the rear swings into action and undoes the front preloading, which you then have to correct with the steering wheel. Unnerving, but completely normal for a truck.
Two points worry me:
Your liquids
your subframe
French law imposes baffles in tanks over 100 litres; even having 100kg sloshing around can upset the way a truck behaves. Your tanks are right above the axle, once you're in a corner all the water is on one side of the truck, that's going to make a difference. Every corner, dip or pothole throws your water to one side or another, compressint one or other spring. Compressing springs randomly changes the geometry of the axle configuration, causing wander. It's the nature of leaves, they're not rigid. Have you tried the same corner or poor road with tanks in different levels of fill?
You re-bushed the anti-roll bar at the back, but as you have no direct link between the anti-roll bar and the actual load that's not going to help.
You've chosen the "semi rigid" subframe, and because the cab is glued to the box you're rigid up front, not at the wheels where you need it.
When a wheel dips into a hole it takes the chassis with it because of your super roll bar bushes. But the chassis separates from your subframe to do that, leaving the little springs to close the gap later.
That "later" is undetermined and can be put off further depending on the state of the road.
In a corner your load is pulling hard on those little springs, which are pulling on the chassis, which is pulling on the anti-roll bar, which is in turn squishing the outside tyres on the corner.
You can see that the tyre is only a small part of the problem, thus changing them only makes a small difference...
My suggestion would be to try replacing your little springs with rigid steel tubes, at least on the four closest to the anti-roll bar. drive the same road with empty tanks, half full tanks, and full tanks and note the difference.
If you find that makes a difference, your little springs were way too soft, you can start playing with spring combinations and bellville conical washers until you find a compromise that you find acceptable.
For the front you can improvise a panhard rod or woblink to help with the sideways precision, and add some swingarms if tramping is an issue. Or you could just live with it; once you're on your RTW you won't feel the pressure to drive it like a car, and everyoune behind you will just have to wait
Hope I've given you some pointers, I'm part of a company that does a lot of suspension design and dynamic calculations for Renault Trucks in Lyon and for a while this kind of issue was my daily bread.
