Religious organisations in Mythras theoretically have up to five levels of membership: lay member, initiate, acolyte, priest, and high priest. Three of these are spell-casting categories as well: initiate, acolyte, and priest. All theism spells are categorised as one of these three levels, and the level categorisation is based on the spell, not on the religion. The different levels of membership control the size of your "devotion pool" which is the number of magic points out of your total pool of magic points that you can devote to casting theistic miracles, which each miracle taking up 1 MP from the devotional pool (e.g. an initiate with 12 POW can devote three magic points to their devotional pool, allowing them to cast three miracles).
This adds an extra layer of complexity when you're designing religions, as you have to keep track of the level of the spell when you're putting together their spell lists, otherwise you run the risk of accidentally creating a religion in which initiates or priests or whoever don't gain access to new spells with their initiations. Considering the limited suggested number of available miracles per cult (up to 1d6+3 total per cult in a high magic campaign), this becomes especially difficult.
Having created my fair share of cults and played a fair deal of Runequest 6, I'd make the following observations:
1) Most of the published campaign worlds don't actually follow the listed guidelines. Mythic Britain uses a totally different system for Christianity that offers 4 miracles per interceding saint you invoke (with some overlap between saints), and has ten or twelve saints in the corebook. The Taskan Empire and Shores of Korantia have different cults offer between 3 and 19 miracles each, depending on the power and prominence of the cult. Classic Fantasy uses some other system entirely that involves three ranks of spells and limits the number you have access to by INT and your level. Monster Island is the one rulebook-following exception, which grants three initiate spells, two acolyte spells, and one priest spell per cult. Some of the top-tier miracles priests get access to are a bit shit tho' (one is "Rain of Fish").
2) Some progression or development is necessary to keep PCs committed to cults. If they can just dip in casually, become an initiate, and learn every possible spell (or at least all the good ones) right off the bat, they're not incentivised to engage further with the cult. Keeping the really good stuff for higher levels of initiation gives PCs something to work toward, so any proposed simplification or solution needs to keep at least two levels of access to spells, and possibly more.
3) I used to think it was feasible to run games where any individual PC might belong to many cults, but after playing RQ6 for a while, I think because of the slow acquisition of new skills and the way devotion pools work, most PCs are going to belong to 1-2 theist cults maximum (and probably one of those will be as just a lay member). That's assuming you even open up the possibility of characters becoming priests if they don't start with access to theist magic (this is something the rules-as-written discourage, but that, once again, is widely ignored in practice). So, if we assume that most PCs are going to belong to 1-2 cults, and probably only one as more than a lay member, a certain amount of depth should be available so they can feel like they're progressing through it.
To simplify the process of creating a religion, my proposal is fairly simple, and really represents a rationalisation of what I see people doing online when they homebrew cults. That is simply to calculate the level of membership required to gain access to a spell on the basis of each religion, rather than the spell. So initiates of one solar religion, acolytes of another, and priests of a third each gain access to say, Sunspear at different levels of membership. This allows greater customisation of each religion, and I'm surprised it's not the default.