Shireneko
since he forgot his past memory his imagination my limit his potential, u should make it like the mc has only bits of his memory not totally erase like this one, like mc only remembering the novels/anime/manga (even a little bit of those memory) he read/watches. With that he can make interesting combo to insert in his card magic.
the cards are basically just a gimmick.. he should concentrate on using his overpowered sword and building a technique instead.. or combining both, such as body reinforcement magic, flame sword, and the lightning spell godspeed or something.. then become an overpowered level 1 swordsman with those three cards..
Nice creativity, at low lvl it might look useless since other mage can just spam low lvl magic but his magic will shine higher level since a level 5-6 magician can partially destroy a city but takes too much mana and extremely long chant and his can just be stored and spam with no chant which is quite broken as long as he stored it before hand.
Depending on the limitations you have it can become a very strong skill, for example, is the only limit your imagination and amount of magic power? Can he put mana on the card and instead of using it all in one potent attack, can he launch weaker attacks multiple times? How many cards can he have? Can he use cards to create passive abilities? Will he get to the point where he can create cards with just mana or store them in his body? Can he create any kind of magic as long as he has enough magic power? Can he use the cards to create matrices, machines or golems? There are many possibilities for this type of power