If nothing else, you seem to have missed that there are two more optional dungeons unlocked as soon as you clear the game. Akademia and the Twinning (unlocked via sidequests that start in the main cities) both drop the same i430 accessories as Amaurot, and you'd earn tomes for completing them that you could then spend on getting those few critical pieces you needed.
50/60/70 roulette also gives a generous amount of currently-Goetia tomes, if I remember correctly.
