You're paying Hill Giant mana for a creature with just half the combined power/toughness
In addition to its rarity, Arthur is quite strong, with an ability that is likely to be leveraged by many serious Lorcana players.
Herald's Horn Herald's Horn is yet another rock that only fits in creature-heavy typal decks but, as one of the best typal support cards, it indeed fits like a glove in those
As loops get more complicated, the chance of making a mistake gets higher, and it's more and more important to be very clear about everything that's happening, ideally keeping track on a notepad for the first iteration in order to make sure you haven't missed anything
The best time to play your first Magic game is now
The one big change I could see is to switch up the exact merfolk that you choose