I think the basic flaw that replaces Dark Secret pre-schism is the Pagan Major Flaw (i.e. does not get along with other religions) - the Diedne magic is especially tied to a strict "pantheon" that makes the Bjornaer ancestor cult look like laid-back hippies.
Pre-schism, their religion demanded secrecy beyond what other Cults would consider reasonable. Post-schism, the theology has adapted to stealth... but only to a degree (Pagan becomes Dark Secret).
For a Cult, I recommend several "Paths", as with the other Cults - perhaps tied to the lunar circle or other natural cycles - give them a cool name. Example : Songs of Ages-
THE SONG OF SPRING : Dedicated to healing and growth. Not entirely "gentle" - Spring also a time of storms....
EXAMPLE MYSTERIES: Minor Purifying Touch, Mythic Herbalism, Fertility, Whistle Up the Wind (Affinity Creo, Life-Linked Spontaneous, Unaging)// Defficient Perdo, Vow, Necessary Condition : Plants, Incompatible PeCo, PeHe, PeVi, PeAn, Animal Companion, Compassionate, Duty-bound, Weakness : Children, Sacrifice (physical body part etc.), Deleterious Circumstance : Have harmed another in the past day, Major Malediction : Cannot eat meat. etc), Hatred, etc.
THE SONG OF SUMMER : The time of plenty which allows for great projects, collective effort and introspection - the fruits of labour of the other seasons. This path are the seers and moral guides of the Cult.
EXAMPLE MYSTERIES: Mystic Correography, Divination, Communal Ritual (cast Spont magic collectively as ritual), Theurgic magic, Dream Magic. (Special Circumstances : Other Diedne, Cautious Sorcerer, Piercing Gaze, Affinity (Cult) Lore) etc.// Optimistic, Dutybound, Proud, Overconfident, Defficient Muto, Incompatible PeIg, Weak Magic Resistance : Cold, deleterious circumstances : No sunlight, Restriction : No fire or sunlight, Vow, Temperate, Pious etc.)
THE SONG OF AUTUMN: The time of change, of harvest and of preparation for the dark times. Associated with change and action - a very physical path.
EXAMPLE MYSTERIES : Shapeshifter, Nature Lore, Enchanting Music, Sorcerous Music, Whistle Up the WInd (Cautious Sorcerer, Life Boost, Affinity Muto)// Short Ranged Magic, Chaotic Magic, Supernatural Nuissance, negative side-effect, Twilight Prone, Reckless, Poor Memory, Lustful etc.
THE SONG OF WINTER: The dark time- one for watchfulness and survival at any cost. A time to come face to face with the darker things in the world...
EXAMPLE MYSTERIES : Faerie Sight (Second Sight and see in darkness), Magic Sensitivity, Sihir, spirit familiar, Spirit Magic or, for a darker view, the Goetic Arts (Puissant Perdo, Special Circumstances : Bleeding, Death Prophesy) // Defficient Creo, Dark Fate, Painful Magic, Plagued by Supernatural Entity, Enemies, Blatant Gift, social handicap-distant, Sacrifice a Hand, Necessary Condition : Bleeding etc, Mute etc., Proud, Higher Purpose, Weak Magic Resistance : Human Contact...)
As a rule, variety is better and makes for more interesting internal dynamics, more interesting characters and more interesting sagas.