The entire concept of threshold and how to read the numbers on the cards is somewhat vague. I mostly ignored it and accepted cards just sometimes didn't trigger and that I'd have to make peace with that. Same with high roller, I guess sometimes my cards just do some extra damage, not entirely sure how much or why.
Hand size increase feels kind of necessary if you're going for shadow seals. I'm sure you know better than me what happens when you apply two to a card, I haven't tried it.