If the due date and the reminder days are recorded on the item, which it sounds like is the case, you may be better off creating a site workflow that would query the list to find the items that should have reminders sent.  You could then schedule that to run at 9am each day. 

Hey Anthony Parker‌,

totally agree with Brendan! If you need help setting up the site workflow have a look at this article:  

Best regards,

Jan

I think it is better to start another workflow from the main workflow, you can schedule that workflow based on any conditions. For an item this way is better.

Hi Anthony,

      For this scenario I would suggest you to go for a Nintex Site Workflow which can be scheduled to run at a desired time as per the requirement .

Scheduling Window : 

WFName : Select the WF you want to schedule 

Schedule Start : When you wanted to run this workflow. (date & Time)

Repeat Settings : Number of Iterations.

Schedule End : When this workflow wanted to End.

Thanks everyone for your ideas. I'll investigate Brendans solution I think. Schedule the workflow to run at 9am for items with Reminder dates less than or equal to today. For each of those items start the email workflow and update the items reminder date based on the number of days set in the interval.