The way I would do the unlocking is something like this.
Once Soulbound reward cards go out of print they become 'starter cards' so bots can't destroy the economy even more. In order to unlock and have them similar to normal cards, the 2x DEC burn value needs to be burned as it is now to make them usable on land + 1 Card of the same type needs to be burned.
Legendary needs another legendary card burned, Rare Summoner needs another Rare summoner to be burned maybe even from the same Splinter to keep it even. This would prevent further mass inflation of cards and actually give them some value to the point where they won't be dumped on the market.
If anything is needed, it's a Card Burn Mechaism so holding cards isn't a guarantee to see their prices in a race to the bottom as it is now.