Theme recommendations that use media: tags in RSS feed

Did you try a plugin?

https://www.ads-software.com/plugins/wp-feed-post-thumbnail/

I have seen that.

The write up says
WP Feed Post Thumbnail is very lightweight and only adds two small options under Settings -> Reading.
But I have no idea what the ‘added to small options’ are??.

I am always wary of installing plugins, just incase no going backwards. I suppose backup or clone or stage my wordpress site

Yes, always test new plugins on local copy of the site and if it works as expected, use it in your site.

But I have no idea what the ‘added to small options’ are??.

I think it refers to this:

https://ps.w.org/wp-feed-post-thumbnail/assets/screenshot-1.png?rev=1582768