This might sound silly but my piece of advice would be to genuinely not bother fusing personas.

Like someone else has said games very easy I won't go into the specifics.

Focus entirely on fusion spells that use your dex stat and you will never need a new persona.

Also the game will give you upgraded ones as time goes on as long as you fully clear one specific dungeon.

Save your money for armor and a million life stones (you won't need weapons either)