You cannot add custom action on the related list.
Review the bellow idea : Allow quick action on related lists
However, you can use a custom button on the related list (Display Type : List Button).
There is a brand new component Dynamic Related List – Single (Spring'22) that is perfect for this.
While viewing the lightning page you wish to edit, click the gear
icon and select Edit Page
From the ...
When the standard detail page doesn't display when clicking on a link to that object, the most likely reason is that the standard View action has been overridden for that object (standard or custom)