Best way to create filters for a product that on the front-end of the shop?

[Vcns]

Let's say I want to create filters for the customer to filter out products when browsing.  For example, lets say I want the filter option available to the customer to be "Franchise" or "Series" and it's a graded trading card slab.  You have a graded card that belongs to a franchise or series "ABC" and it also belongs to franchise or series "XYZ" as well.  Maybe XYZ is the sub category of ABC. 

So if I set up variants and I create two entries for it, one being ABC and the other being XYZ, and it's a single item, well the problem I run into is that if I have the amount available as 1 for ABC and 0 for XYZ, it will show XYZ as out of stock.  If I change XYZ to 1, well, now I have two items in stock when it really is just one.

Within Search & Discovery when I go to create a filter, the options are the variants that I create which ends up being a problem if you try to have two for the same product, or other values that don't quite fit.  Maybe if I could add custom category metafields to "Non-Sports Trading Cards" but I don't see an option to add a custom field to it.

What's the best way to go about doing this?

[Vcns]

Thanks for replying! Like I mentioned in the initial post, that creating metafields seemed like a solution. But how does one go about making custom ones to an existing category?