I wrote this post a while ago, weighing the pros and cons, but I think it is still very relevant:
I cannot stress enough how hot the real estate market is - whether you’re looking to rent or buy. The good listings don’t last long (unfortunately for buyers/tenants). So please bare that in mind while apartment hunting.
For rentals I’ve never found a realtor to be necessary - you’ve pretty much listed all the useful websites for hunting (beware of scams though, especially on Kijiji).
When I was new to the area I found Reddit to be a good source of recommendations for apartments, shops, services, etc.
https://www.reddit.com/r/waterloo
https://www.reddit.com/r/kitchener
Here’s an apartment review thread:
https://www.reddit.com/r/waterloo/comments/ofoc0c/apartment_building_reviews_master_list/
Good luck!