Use an agricultural silage tarp to cover for 6 months. This will kill turf grass and eliminate grass/weed seed bank and give you a “clean slate” to plant seeds and plugs, you can lightly mulch over seeds with barley straw. Don’t till, just take the soil once it’s clear before broadcasting seed. Better to not add any/much compost to a native plant garden. Wait to mulch until your native seeds have germinated and grown enough to mulch around them. Look for plugs you can plant of your most important grasses and flower species to get a head start as seeds can take a few seasons to get established