Hope you find a cycling partner for your trip. I was trying to figure out the background in avatar: I lived in Toronto for over 20 yrs. (did cycling there too).
Are you taking the waterfront bike route east towards Kingston? We've done different trips for that piece. Prince Edward County area (Brighton, approx. 135 kms. from Toronto, etc.) is lovely. It is strange to see dairy cattle by Lake Ontario after leaving the Toronto, Pickering, etc.
We have also done several self-loaded different bike trips to from Toronto to Fort Erie, with stayover in Niagara-on-Lake in summer and fall. (For those unaware, it's approx. 120 kms. 1 way to Niagara-on-the-Lake + additional 25 kms. to Fort Erie, ON, right by the Canada-U.S. border. As you might now there is a campground near Port Dalhousie by the bike route there. But make sure you book many wks. in advance. It's heavily used by RV travellers.
So haven't gone all the way around Lake Ontario. I've heard interesting stuff on the U.S. system by using routes close to the canals. My partner did cycle on that side (or was it Lake Erie?) into New York State.