In my mind, just getting out of Toronto is going to be the worst part, but the part you know best. It is mostly flat and generally downhill with(usually) tailwinds. You might encounter a few rolling hills at some points. You might want to Google the 401 bike challenge. My son has done it twice and the route it takes is the most bike friendly
Riding into downtown Montreal can be done on the Lachine canal bike path. That was my commute route for 25 years. I never tired of it
5 days? you don't even need 5 days of clothing if you stay in hotels. You can wash your kit (and underwear) every night









0every night if you stay in mote