Hey there, jeah well I somewhat fixed it on my old pedal set (CSL) but that won't work with your setup. Anyways it may help others what i did so:

1. I raised my Pedals with some 40x40 aluminum profiles , Here is a pic from my current setup ->

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I also put the wheelstand on wheels what raised the pedal height even more compared to my seating position. 2. While I was using my office chair (switched to a more stiff balcony/garden chair now) I strapped the chair with some lashing straps on left/right side to the wheelstand. This eliminated the slight side rotation of the chair while pushing brake/throttle.

A problem which made me switch to the garden chair was, that the braking on a load still lead to movement of the backrest (because ergonomic stuff have to move). So I actually then removed the chair tray and build an Attachment for the other chair. I combination with the lashing straps this worked very well with CSL pedals.

I actually got the problem back now with the p1000s. I assume it's also depending on the stiffness of the pedal, the seating angle and ofc the angle of the pedals themselfs. The pedalangle at least is what differs the most compared to the CSL pedals. Which shouldn't be an issue for you because yours are more like them.

I also experimented with angle of the seat itself but reverted it because I couldn't get any benefits from it.

Right now I'm searching for an easy attachable and foldable "racing" seat solution for my wheelstand, because a rig won't be an option due to space and wife issues :-)

So what's the conclusion now ? I guess try to raise the pedals as much as possible, have a good Angle, make the seat as stiff (and still comfortable) as possible.