I watched the vids again and it looks as though you had less body movement when you were leaning forward. Again, adjust the seat back more angled away from your back so it doesnt hit you, and lean further away from it when you go over any rough ground.
You need to set the seat weight about 10kg or so more than you weigh, or just enough so it doesnt bottom out most of the time. Setting it at 130kg or what ever is too stiff. I weigh about 85kg and the seat is set at around 95kg.
Below is a photo of where my deck springs are set. I have about 1.5" more for adjustment. How much do you have? Try adjusting that nut all the way to the end, both sides of the deck. If that doesnt work ask your dealer about shortening them. It will be a pita to do so let the dealer do it if you go down that road. May even need to find a shorter and or weaker spring. I would not take them off altogether!
Get yourself a low pressure tire gauge. Unless your dealer used one, you have no real idea whats in there. Last time I let my dealer level my deck they set one tire up around 24psi and the other 18 or so. I asked for 10psi. Dont even get me started on all that though!
Try holding the levers further down, like around the first bend. That will give you less leverage so therefore probably less chance of jerking. Also, use a lighter touch.
The above should sort your forward/back, up and down but going front on over ruts like your doing in the vid will be rough no matter what machine of similar size, Ferris excluded.
How did those ruts get there? The place has either been mowed in both directions or possible circular. Seriously try a different direction. Think of the union jack with the 4 different angles. Doing angles doesnt add much time and Im sure youd be able to make it up with a faster speed. Next time you go over a speed bump, try hitting it at an angle. Its much less severe then hitting it front on as only one wheel has to absorb the shock at a time.
Id like to see vid of you driving the Dixon over those same ruts. Id also like to see a side by side comparison of speed between the Dix and the SZ. I wonder if your actually going faster than you think.
Below are a few pics of a property I just picked up. I DID NOT do whats in the pics. I got the job because of it. Every one of those lines is a rut from being mowed in the same circular direction for god knows how long. The terracing is insane. Point is, no way I can get any real speed on this place as the deck will bounce badly and is really not good on my back. Point is, I know its rough so adjust my methods accordingly.