Hi Steven, 

Have you looked at using Actionable Messages? ( Lights! Camera! Actionable Messages!! ). This presents your task options as buttons within the task email. 

Weird. Try this one: 

https://community.nintex.com/blogs/NintexProductBlog/2017/10/04/lights-camera-actionable-messages

Here is the help files link as well for more of the configuration and prereq's info.

https://help.nintex.com/en-US/o365/o365/O365WorkFlow/CreatingWorkflows/Actionablemessages.htm 

Turns out, I was using this already. The problem is that I need this to work with On-Prem and email clients other than Outlook.

To add more detail, my client essentially doesn't want to type "accept" or "reject" and wants HTML buttons in the email. So my only solution ideas so far are to have a webservice that creates the email for him, or figure out how to update the workflow using the Nintex API