First a point of clarification: typical Thaumaturgic focus items don't give you a choice between bonuses to Conviction and Discipline, they give a choice between bonuses to Lore or Discipline. (Conviction and Discipline bonuses are the options for evocations, not thaumaturgical rituals.) And to clarify further, the bonus is to either Lore or Discipline, not Lore and Discipline as you have it written in the question.
That said, when creating a thaumaturgic focus item for crafting rituals, you can choose either a frequency, or strength bonus (see YS p. 280, top of the second column) instead of a Lore or Discipline bonus. However, notice that such a focus item would only be useful for creating enchanted items since strength and frequency are irrelevant to the creation of focus items (and the book even says that you can't use focus items to make other focus items).
Finally, it's worth noting that taking Ritual with a specialisation in crafting won't let you create more focus items and enchanted items than you have focus item slots and enchanted item slots:
Wizard characters get a number of “slots” for different kinds of items, under the assumption that there is a practical maximum of items that a wizard can make and maintain at one time. That number rises via character advancement (and the purchase of the Refinement ability—see page 182), allowing the wizard to either possess more items or create stronger ones. —YS p. 278
To get more slots to fill with your creations, take Refinement during character advancement.