I saw one or even two mods that addded backpacks

But how about
Use Valheim+ to raise the card slots (up to 30*8)
Use Valheim+ to reduce the effect stored weight has on the card
Use "Get off my lawn" to give your Card and Boat 999999 Hp Life

But be warned, raising the slots leads to a unsafe savegame, if a ninjapatch disable the mod you will loose all stuff in these extra slots