ACF: How to programmatically Update Flexible Content Field?

I want to be able to create sub fields inside a flexible content field.

My setting is this one:

menu_item_field is a flexible content field that contains:

1.1 menu_item_parent (text)
1.2 menu_item_child (repeater field)
---1.2.1 menu_item_child_title (text)
---1.2.2 menu_item_child_text (text)
---1.2.3 menu_item_child_img (repeater field)
------1.2.3.1 menu_item_child_img_item (img)

This is the structure that results of the above setting:

Group One
Item 1 title
Item 1 description
Item 1 img 1
Item 1 img 2
Item 2 title
Item 2 description
Item 1 img 1
Item 1 img 2

So far I’ve been able to insert only the Group field and I wanted to know if it’s possible to do more complex stuff.

I did it using:

add_row(
'menu_item_field', [
'acf_fc_layout' => 'menu_item',
'menu_item_parent' => 'Group One',
], $postID)

The above code returned only the main group titles:

Group One

I know there’s update_sub_row()

but I haven’t been able to successfully use it.

My main goal is to be able to edit or create new fields using a custom UI in the frontend through AJAX.

I prefer this instead of acf_form().

So, my question, is it possible to achieve what I need and what would be the proper function with the above settings.

Thanks.