WP Simple Fields – Single repeatable field inside repeatable field group

But it seems only field groups can be repeatable and not a single field inside.

Can’t you make a dummy (image+something) group for this? Then, hide the something with CSS.

unfortunately I cannot change the plugin for this project

In the backend, if another plugin uses the same meta names -let’s say Simple Fields and ACF- both plugins will show mirrored values (in non-repeatable cases). Although, I don’t know if saving the post has unintended consequences. This is useful to make tests in a clone site before migrating from one plugin to another. In the frontend, using get_post_meta covers any plugin.

So do I really have to extend the simple fields plugin to accomplish this?

You could use ACF or a handmade solution to create another meta box.

there seems to be no option to make conditional fields for the cms only

ACF has that. Anyway, in this situations jQuery is our friend.