Holland Michigan - reportedly has some downwind routes, locals report that rebound off of seawalls can be significant, rip currents also apparently merit attention.
Seaway Island - Mitchell's Bay 13 miles (Canadian side of lake)
St. Clair River
River flows from Lake Huron to Lake St. Clair, border between Michigan, USA & Ontario, CA. With a south wind building waves against the current it can work a bit like the Gorge or a tide race. You can also do laps surfing with the south wind waves going upriver then using the current to go back to lap starting point. Sarnia, CA foil laps
Port Huron (or Sarnia, CA) downwind, downriver 12 km sup foil video
TCSurfski.com Lessons, Sales, Rentals, Local knowledge. Traverse City surfski is not just a local outfit, they keep up with surfskiing and downwinding on a comprehensive basis. Very knowledgeable. Surfski on the Great Lakes Podcast
Mosquito resevoir: good downwind action with deep enough water for decent downwinders
Pymatuning Lake: very deep water, well situated for downwinders
Alum Creek Lake (Columbus): deep water for speed, but short for an out and back
Lake Erie from DougMar on surfski.info “…an incredible lake, DEEP near Cleveland, kinda shallow on the West end of the great lake, but Cleveland areas have tons of miles of nice Westerly fetch, and good sized rollers (12-14' in many 20 knot westerly blows). Best thing though, is hooking on to a roller in Cleveland, and ending up in Painsville 30 miles away!…”