If you can hang on to your green rings until the Double Materials Week is available you will get maximum value from them.
Is there certain materials, items & ingredients you are particularly chasing in Greece?
Have you checked out the relevant Wiki pages in relation to the Older Segmented Stories that are available in the shop to purchase for 500 gems?
I'm not sure whether it is worthwhile you looking at the Older Unsegmented Stories that are available in the shop to purchase for 100 gems as well....as they might not have enough of the region specific materials, items & ingredients that you are seeking.
You can check out the relevant Wiki pages here for all the different Older Unsegmented & Segmented Stories here
I have also done some spreadsheets in relation to all the camp equipment that is available in the Older Unsegmented & Segmented Stories here....you can check them out if you are interested.... I'm due to update them again a little bit....which I will finish soon....unfortunately I've had a few medical issues slowing me down a little bit lately