Hello,
this may sound partially like a luxury problem, but I would like to add a suggestion: Rares should be exchangable for other rares of the same type for a certain fee. This would mean that you could go to the bazaar or an area you would unlock upon a certain score/with a quest and trade stuff. This stuff would be one weapon for another, one Squid for a Pharao, one Shard for another and one Nymph for another. Upgraded cards could only be traded for unupgraded cards.
With that system new players could exchange weapon (i.e. a Arsenic for a Pulverizer in order to farm FG). Also Shard decks would be more accessible. And lucky players could exchange Nymphs for some they don't have/they would like to have. This would also fix the problem, where one player has a double digit amount of one type of Nymphs and not a single one from another type. The total amount of Rares would not change.
Prices could be 100 for the weapon exchange, 200 for Squid/Pharao, 500/1k for Shards and like 10k for a Nymph.