I like this idea, but I would decouple the acquisition of Rune Points from the acquisition of spells. You get a spell when you get a Rune Point, but you should be able to get spells without POW sacrifice as well.
For example: at any worship ceremony, there may be a group of members who undergo a minor quest to acquire a rune spell. Several such groups may be active at the same time, particularly at larger and more major ceremonies. The number that can take part is limited, and priority is given to those who are sacrificing POW to gain a Rune Point at the same time. If there are spare places, then other members (in order of cult standing and reputation) can participate and acquire the spell without sacrificing POW. If the spell is variable and they already have it, they can gain the ability to cast it 1 point higher.
So it's up to the GM to decide on, or the character to earn, the availability of additional spells like this. Eventually you might end up with several Orlanthi all looking the same, able to cast the same variety of big variable spells, but it will take a while.