When travelling through Canada you have Immigration to clear Once on the outward and twice on the return.

If you want a Thin Flat bed with Great Lounges with the opportunity to enjoy a pre-flight dinner in YYZ then go BA.

If not then , AA 767 ORD-BRU and enjoy the FL at ORD.

Avoid MIA and YUL , also look for BA 747 services on YYZ-LHR-LHR route, nicer upper-deck and NNCW hopefully.

Personally I would do YYZ onto a 747 on the outward and into ORD on a 767 on the return.

Finally look at Train Travel in Europe and LCC, although you can hit some cheap fares on BA occasionally. I also believe there is a train from DUS to FRA Airport which might connect to the 767 FRA-ORD.