Set up Task's Due time with automation with Slack issue

Hello, I have have a set of about 10 daily tasks that need to be completed at different specific times during the day and these Tasks and their times are repeated every weekday. I Monday to integrated with slack already, but I wanted to send automated messages if a task is not started or completed at the specific time.
I figured out how to make a task Repeat ever day by adding a Start date and adding specific Time to each task’s start date . The task repeats daily perfectly fine, But the specific time goes a a random time the next day, like 7pm. For example : Lets just say its Tuesday and I have Task 1 is to call back clients every weekday from 10am -11am and I have it set to repeat every weekday at the same time. Once completed, the task will Reoccur on Wednesday but the due time will change to 7pm for some reason, which of course messes up the automation.
I also tried making a separate column just for a Due time, but I cant figure out how to set that automation. The rules only seem to work with Due dates as for a s I can find. I hope this makes sense. Any ideas?? Thank you!!!