This was useful for me in customizing the response: https://jss.sitecore.com/docs/techniques/extending-layout-service/extending-layout-service-overview.

But, be very careful with it. It may break other JSS functionality here if you change it too much, which it looks like you want to. Adding fields is alright but changing the item serialization may take you in to holes you can't dig out of

This was useful for me in customizing the response: https://doc.sitecore.com/xp/en/developers/hd/latest/sitecore-headless-development/extending-the-layout-service.html.

But, be very careful with it. It may break other JSS functionality here if you change it too much, which it looks like you want to. Adding fields is alright but changing the item serialization may take you in to holes you can't dig out of