Hello Philll, nice to hear your method which sounds good for clearing weeds.
I am concerned that your ‘suppressant’ is plastic? is it mypex for example? I would not leave any synthetic material in the soil, it will interfere with movement of soil life and roots for a very long time. Unlike say cardboard which just breaks down and is eventually worm food. If much of your grass is couch, you may need to use some cardboard, if not the compost should provide enough light-suppressing cover.
Most nutrients in compost and older manure are not water soluble, they are distributed by soil life and become available in warmer conditions.
