How to modify the image output for just one content type using Responsive Images?
The responsive image module creates these with a twig template called:
responsive-image.html.twig.

You have two options to change the output.  You can either copy this template from it's location under core/modules/responsive_image/templates to your theme.

Or you can use the hook_preprocess_responsive_image() in a module or your my_theme.theme file.

Basically, you will need to change the values in the source_attributes object. E.g. grab the value of the src attributes, delete it, and add your srcset attribute with the value of the src attribute.