I've finished the 1st and 2nd (√A) seasons and I wanted to start with the manga. So I want to continue it from the part where the anime left off, since I don't want to waste time reading what has been already done in the anime, assuming all the episodes are canon. If they are, which chapter should I start from? Or should I start from the beginning? And one more question, is it a weekly or monthly published manga?
I also suggest you start from the beginning, not only because they changed some things storywise but because a lot of the small details of the story were omitted in the anime. Those small details will help you to understand the plot better but also play a big role in bringing the characters to life.
Also if you want to watch the Tokyo Ghoul Re anime there are references and parts of the story that you wouldn't be able to understand well without reading the manga. For example, in the Re anime, Tsukiyama makes reference to activities such as
playing a match of Squash (a sports game) with Kaneki which was in the manga but omitted from the 1st season of the anime. The fact that Tsukiyama wanted to play Squash also reveals details about Tsukiyama's character: He believes exercising is essential but more importantly it emphasizes Tsukiyama's obsession with contrast in things (food, emotions, activities) since he wants to do something upbeat like playing sports before going to a relaxing cafe. After that, the manga also shows that he likes to drink a cold beverage first and then a hot one which reemphasizes the theme of obsession with contrast.