Strimmed now I can see where I am at

[Rose willow]

So a good friend very kindly volunteered a few hours work and helped me strim the foot long grass down so I can see what’s what. Was doing it for about 3 hours and I think managed to strip half the plot. Found an old bonfire site with numerous burnt tins and glass bottles so sorted that. Now it’s strimmed I can see that the whole site is covered in couch grass. My initial plan was to tarp over most of it and leave to die this year and just concentrate on small area. I have 2 pallets I have broken up to make about 3 simple raised beds and I was intending to put layers of cardboard down my square wooden bed on the top and put in some bought multipurpose compost mixed with some top soil if possible then plant stuff in that. As it’s couch grass would this still work? Had to give up after a few hours as the strimmer hot some hidden metal and the plastic bottom bit shattered into pieces. Whoops!

[ameno]

The black plastic approach will work on couch grass, but it takes longer. One year may not be enough.
With couch grass, it's also vital that it really is plastic or tarpaulin which you cover it with, not that woven weed suppression membrane. It needs to be a continuous sheet of some sort, as couch grass can and will push its stolons through the weaving of the weed suppression membrane.

As for your mini raised bed idea, that should work as long as you make it deep enough. Couch grass likes to have shallow roots, so if you bury it deep it tends to just die. A double layer of cardboard would likely help with that.

[Rose willow]

I didn’t realise it would take that long to die. And I have put weed membrane down as it was all I had. Good job I asked. I will have to replace it I guess. I really don’t want to put weed killer down if I can at all avoid it.

[Plot70]

I had loads of couch grass on my plot when I took it on in December 2019.
I dug deep and pulled up quite a few wheel barrow loads of roots and many slabs of wet clay with some other kind of grass that had fibre roots that held onto the soil like the stuff they sell as turf for football pitches.
I built a raised bed over a patch of mugwort from the clay blocks and filled it with couch grass roots and sealed it all down with steaming hot horse manure with a slot in the top that got a layer of compost starter pellets washed down with the 5 gallon shed latrine tank.
I capped it with top soil and planted squashes in it.
I got a very good crop including some butternuts.
There was almost no couch grass sprouts and now I have a massive bed full of lovely compost with quite a bit of fibre mixed in.
I am thinking of using the compost and digging and planting the clay below that contains suffocated mugwort roots and anything that washed down into it throughout the year.
Couch grass roots can impale dandelion roots and some plastics. They can be killed by exposing them to strong sunlight for a week or so if you have got a space.

[ameno]

I didn’t realise it would take that long to die. .

You'll find that most of it will be dead after one growing season, but not all of it will be, and a sufficient amount is likely to remain that it could repopulate is left unattended.
One growing season may be enough to kill enough of it that you can just did out what remains, though.