Once i get everything from the card trader, what can i do with gold?
Shouldn't be still usefull? Maybe to buy card tickets or something like that...
I think upgrading to glossy/prismatic take stones, not gold?
They'll add more Card Trader cards, so you'll be able to spend more gold eventually. As for orbs, they can be useful for gathering event tickets (like the invitation letter for the tag duel) or mindless grinding using Auto-Duel. If you come to the point where all your characters are max leveled and you don't need to grind for event tickets... then you might have been playing DL a little too much lol.
I highly doubt you will ever get everything from the trader, even if you get 3 copies of each card he sells (which would require a ridiculous amount of gold and other tokens already), you still can upgrade all the cards to glossy and then to prismatic, which is even more expensive.