You might want to reconsider the idea of tier-based ritual casting. If you take a close look at them some rituals are very useful, but actually duplicate the ability of specific magic items. The Exodus Knife, for example.
If you're taking out skill rolls from rituals, then you'll have to raise the cost of casting them. If you don't, then you're removing a significant barrier to spamming certain rituals. You can also keep skill rolls but allow alternate skill use, which can eliminate the issue of the skill barrier.
