Until V12, I was able to create new variants (product.product) by importing them via excel and mentionning the attribute value(s). Since V13, I can't make it work anymore.
So, what I do now, is to define the product attribute with "variation creation mode" = instantly.
Like this, when I import my product.template, all the variants are automatically created.
I then export the variants in excel, with the attribute values, and in excel, using VLOOKUP's, I fill in extra data (like you: default_code, sellprice, costprice, weight, barcode, etc).
I do not add a column in the excel with the attribute value. So, in your example, I would remove the column D.
To disable the variants that were generated but that do not really exist, I import via excel the field active = false.
Finally, I observed that importing the default_code on the variants empties the default_code on the product.template. So, after importing the variants, I re-import only the default_code on the templates.
I hope this explanation helps you.