This can be achieved using 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 in the Epic IM field.
The sub-tasks of each issue inside the epic are shown on that line as well.
Check out an example on how it looks in the screenshot below:
...
Find the below example for reference:
Create respective fields in the sub-task custom field as shown in the below screenshot.
Create respective fields in Epic custom field as shown in the below screenshot.
Refer to the below gif for reference.
