Welcome to the Community, @ac-piffers.
I've got the video that you need on how you can set an automated invoice reminder for your customers. This way, you can gently remind customers about invoice their due dates.
Here's a link that contains the video, which provides the step by step procedure on how to achieve this: Automate invoice reminders in QuickBooks Online.
On the other hand, we don't have the option to set a reminder of the customer's contract renewal date is approaching. If you're referring to adding or tracking the contract renewal date on the invoice template, you can do this by adding a custom field.
Let me guide you how:
To give you more tips about customising sales template in QuickBooks Online, please see this reference: Learn how to personalise your sales forms and add the info that matters most to your business.
I'll be right here to assist you further if you're referring to something else or if you have any other questions. Just leave a reply below. Take care!