1. Replace sacrificed Card's Faith into the Hero
This is my preferred choice. This will allow any 5 stars unit to be fused and transfer over it's Faith. This is quite balance on the design as it's not really that easy to get a "spare" 5 stars unit with the faith you want.
2. Using more number of lower rarity card to reroll
This is still too much RNG and causes more frustration. It doesn't fix anything.
3. Using same Hero (Kai) at any rarity to reroll, your Hero (Kai) faith
Yet again another bad design, you need the same unit for Limit Break. And again way too much RNG involve. Work on releasing more Heroes than implementing too many RNG factor into a mobile game.
4. Option to manually select Faith for each reroll
This can be a very good choice if done properly. It gives players a huge sense of satisfaction when they can finally choose the Faith they want. Learn to please the players and you will get back more revenue than expected. Do not cluttered the game with too many RNG system, the gacha system for unit and equipment are good enough in fact.