Yes, you can set a default grade posting policy for a course. There is an article describing how to do it (along with a description of how all the different grade posting settings interact) at https://community.canvaslms.com/t5/Instructor-Guide/How-do-I-use-posting-policies-in-a-course/ta-p/9...
@mzimmerman has given you the answer you need.
I will point out that once you set that policy for the course, you may have a possibility of it not being enforced. If my recollection of what has occurred to me is correct, if you import an assignment or a quiz from another course, then the grading policy for that assignment or quiz comes with it. So if it is set to automatic in the course you are copying from, then it will be automatic in your course. I may not be 100% sure on this matter, but I seem to recall having that happen to me this past semester.
So, if you import an assignment or quiz, I would check to make sure that the policy is set to manual before posting it.