Or we could simply not have 100 rules for a simple EC and just let it be simple.
Make 30-60 card legal decks, choose 10 cards as the "sideboard", roll 1d10, add the card. As simple as that. There's no reason to add hundreads of exceptions and add confusion.
The ONLY exception that should be made is when the deck already has 6 of the card, since you actually can't add the card, so you reroll. (and obviously you can't add a card to 60 card decks)