Thing one: 50? For just one instructor? Insane.
Thing two: These are grownups. Start entering scores as soon as things are due, and make the gradebook accessible to them. Establish a policy that you have a set time when you look at the submissions and enter grades, after which they will not be changed. If, however, they contact you for help in advance you will provide coaching so they can complete things by the deadline.
Thing three: Is this course a requirement that they are satisfying for a program? Is it a prerequisite for another course? Is it obvious to them how the knowledge they are gaining in this course will help them in their degree programs?
Thing four: Online courses require a lot more initiative to read, and many students are still passive, accustomed to absorbing the words of wisdom in the lecture hall making it unnecessary for them to read much. Add some metacognitive discussion regarding the time it takes them to read, with your own estimate of how much time you think they should schedule to do so. You can survey them in the guise of a quiz by asking such questions and then responding to them. But with 50 in the class, it is unreasonable for you to attend to the individuals so closely.
Thing five: 50? really? Worse than insane. Exploitative.
Thing 1. At the University of Washington they teach a similar online course to 1000 students a semester.
Thing 2. Enter Scores? Scores are automatically entered when they finish the exam.
Thing 3: The class is a GUR science class for non-majors.
Thing 4. Metacognitive?
Thing 5. I'm paid $330 per student to teach this one six week class during the summer. You do the math. I don't feel exploited at all.