20 meters

It was tough but I finished digging 20 meters along the side of the plot, cutting off roots that were crossing over from the neighboring plot. It took some time as I did it all manually with a shovel and a pick axe.

20 meters trench

Now I needed to widen it to a meter, so that I can bury the root barrier sheet there, backfill with soil and still have some margin to continue cleaning roots on my side later on.
The digging and removing roots itself is not that difficult. In fact it is pretty interesting and I could very well do the whole plot by hand.
However, every time I got home from hours of digging, I felt heavy in my lungs and eventually developed a bit of coughing and itch in my chest. No matter how well I used the mask it seems the dirt and stuff somehow find their way into my respiratory system. Especially in there corona times I don't want to intentionally weaken myself so I decided to stop digging for hand and get a backhoe again and finish the job that way.
This is how far I got with the widening before I decided to use machinery.

3-4 meters maybe?

I noticed how gradually more solid and clayey the ground gets after 30 cm depth, and how if I just leave it like that it probably would turn solid.
Also, although it is hard to imagine, the ground is really FULL of rhizomes. I think after removing all the roots I would be left with noticeably less soil than now. So I decided to, after I am done de-rooting the plot, mound up the top soil in rows on top of the hard surface and make my swales that way, with paths in same depth as I have dug now.