I'd love to be able to filter and only see cards that I have 5+ copies of. Am I missing a way to do that, currently?

You can sort your Inventory by that column, but to my knowledge you can't filter by that criteria, at least not on the website.

You can, however, export a CSV of your inventory, open it in Excel, and then use Excel's filtering and sorting. This will allow you to filter out cards you own <5 of first, and then apply multi-level sorting to the rest.

I agree, though, that having a native Deckbox way to do this would be better. Exporting just to do a little filtering is very inconvenient.