in that case, why can't you just keep the game card for now? it's not "unfortunate", it's better in this case.
in either case, the only way you can back up the save to move to another card/digital version is homebrew. though you said you're buying Freakyforms Deluxe, so you could just use that to back up the decrypted save if you're then going to sell the game card off.
if you seriously intend on burning money on a Gateway card, then you can just do that after you get Freakyforms Deluxe and sell ALBW (or something). as for importing the save to a ".3ds" version for GW, I can't help you there, so good luck.