Since you want 3–4 secondary flavour arcs, I’ve built them to enrich character relationships, deepen themes and create breathing room between the larger tension spikes.

Subplot 1: The Confession Exchange

Flavour: Character relationships + thematic depth
Characters: Jules, Tessa, Dr Lila Moreno
Theme: Authenticity vs performance 

Setup
Early retreat exercises require guests to anonymously write deeply personal confessions. Silas claims the notes will never be shared. Later, fragments of confessions begin appearing in unexpected places. Scribbled on mirrors, hidden beneath dinner plates, projected during exercises.Everyone assumes somebody is weaponising the information.

Middle
Tessa initially becomes comic relief and seems superficial, but Jules gradually discovers that Tessa's online persona has left her emotionally hollow. Jules and Tessa begin developing an unlikely friendship. Meanwhile, Dr Lila insists group paranoia is causing people to invent connections that do not exist. The audience begins questioning whether Lila genuinely believes this or is controlling perceptions. 

Resolution
Near the end, Jules discovers many of the confessions were altered before being redistributed. The exercise was never about honesty. It was designed to prove how easily truth becomes corrupted once filtered through other people.

Supports main narrative
Mirrors the larger whistleblower conspiracy
Reinforces the idea that narratives can be manufactured
Gives Jules an emotional connection amid growing distrust

Editorial pacing notes
Place quieter Tessa scenes after major deaths or revelations. They allow emotional decompression before tension rises again.

Subplot 2: Naomi's Obsession Board

Flavour: External conflict + character development
Characters: Naomi, Jules, Ethan

Theme: Truth versus attention

Setup
Naomi secretly begins creating a physical investigation wall in her room using photographs, notes and retreat schedules. She believes Silas is orchestrating something larger. Jules initially dismisses her as somebody chasing another sensational story.

Middle
Naomi and Ethan repeatedly clash. Ethan argues that people see patterns where none exist. Naomi argues that institutions survive because intelligent people ignore uncomfortable connections. As more strange incidents occur, Naomi's board begins making disturbing predictions that later become reality.

Resolution
Jules later discovers Naomi's theories were partly correct but partly manipulated.Someone had secretly planted information into her investigation.Naomi was investigating the truth using false evidence.

Supports main narrative
Reinforces themes of surveillance and manipulated information
Creates uncertainty around whether clues can be trusted
Allows exposition to emerge naturally

Editorial pacing notes
Scatter small discoveries throughout the novel rather than delivering large information dumps.

Subplot 3: Fiona and Marcus

Flavour: Relationship subplot
Characters: Fiona, Marcus

Theme: Reinvention and redemption

Setup
Marcus keeps mostly to himself and distrusts everyone. Fiona repeatedly attempts to engage him despite his cold responses. Initially, readers suspect ulterior motives.

Middle
Marcus reveals pieces of his military history and guilt over intimidation tactics used against innocent people. Fiona reveals that her philanthropy largely existed to make herself feel morally clean. They gradually become two people trying to convince themselves they are not defined by their worst choices.

Resolution
Their bond never becomes a full romance. Instead, they become one another's proof that change might still be possible. One later risks their life for the other.

Supports main narrative
Gives emotional warmth inside an increasingly bleak environment
Contrasts manipulation with genuine human connection
Shows redemption may exist even if justice does not

Editorial pacing notes
Keep interactions brief and understated. Small moments feel stronger than dramatic declarations.

Subplot 4: Victor's Architectural Fear

Flavour: Psychological mystery + worldbuilding

Characters: Victor, Jules

Theme: Control and unintended consequences

Setup
Victor becomes increasingly unsettled by details inside the retreat. Doors appear where he does not remember placing them. Corridors seem subtly wrong. Window placements do not match his original designs.

Middle
Victor becomes obsessed with proving that the building itself has changed. Other guests think he is losing control. Jules starts noticing inconsistencies herself. The audience begins asking: "Is someone redesigning the retreat?" or"Has Victor simply never known the whole truth?"

Resolution
Jules discovers Victor designed only the public retreat. Hidden layers and surveillance spaces were added later without his knowledge.

Supports main narrative
Expands the world without changing the core plot
Creates environmental unease
Makes the retreat itself feel like a living antagonist

Editorial pacing notes
Reveal environmental oddities gradually. Readers should notice them before characters fully acknowledge them.

Narrative Integration Notes
Main story focus:
Jules uncovers the truth behind the retreat and surviving the manipulation.

Subplot function:

Confession Exchange → emotional and thematic layer
Naomi Investigation → mystery engine
Fiona/Marcus → human connection
Victor Architecture → atmosphere and paranoia
None of these should steal focus from the central thriller. Instead, they create alternating emotional rhythms:

Major reveal → quieter relationship scene → suspicion grows → death/event → mystery clue → emotional fallout

That pattern tends to maintain momentum while preventing relentless tension fatigue. The strongest addition for The Echo Chamber is probably Victor's Architectural Fear because it quietly feeds your "reality may be scripted" concept without directly revealing the larger twist too early.