Before choosing a mil thickness, I’d first confirm whether “couch grass” here is quackgrass (Elymus repens). In Michigan it spreads by sharp rhizomes, and chopping a live patch with a trencher can turn one infestation into many pieces that regrow. I would not run the trencher until the strip on the lawn side has been cleared or otherwise controlled, and I would keep all rhizome-filled spoil out of the new bed and compost.

I’d treat a barrier as the second line of defense, not the cure. If you use one, look for a continuous smooth HDPE root barrier rather than short pound-in panels; keep its top visibly above grade so runners crossing the edge can be seen and clipped, make the seam accessible, and follow that product’s burial specification after checking for utilities and tree roots. A very deep barrier with a buried top or a weak overlap can still fail, so depth/mil alone is not a useful guarantee.

Before planting the small fruits, draw or stake the exact edge and leave enough access to inspect both sides in spring and fall. Remove every visible rhizome from the bed soil, and skip landscape fabric—it tends to make later removal harder when rhizomes cross or pierce it. Since this is a long-lived planting, a photo or sample to your local MSU Extension office would be worth it: species confirmation changes the control plan, and they can advise what is appropriate for your site.