Thanks for the additional information and the link to the earlier topic.
It looks like you can create one custom field, and you don’t need to use ACF unless you want to. You can go to the Edit Media page for one of your images, scroll down to the Custom Fields and look for the “Add New Custom Field:” area. Click the “Enter new” button, give the field a name and value, e.g., “Collection” and “nebula stars dust”, and click “Add Custom Field”. Once the field appears in a Media Library item it will be added to the “Name” dropdown control for you. You may need to pull down the “Screen Options” menu and check the Custom Fields box to see the area.
You can compose a gallery with something like:
[mla_gallery meta_key="Collection" meta_value="nebula stars dust"]
I am marking this topic resolved, but please update it if you have any problems or further questions regarding the above suggestions.