Here's how you can achieve this using the existing functionality of Issue Matrix (IM):
Create a separate IM field for sub-tasks.
Specify which columns are relevant to sub-tasks.
Add the new IM field as a column to the Epic IM field.
The sub-tasks of each issue inside the epic will be shown on that line as well.
Check out this simple example of how it could look in the attached screenshot.
Please find the below example for reference:
Create your respective fields in the sub-task custom field as shown in the below screenshot
Crete your respective fields in EPIC custom field as shown in the below screenshot
Please refer to the below gif for reference to achieve your target.
