Spike: Graph + CalDAV Service

Summary

This is the strongest fit for the actual requirement set.

Build or adapt a small service that uses:

Microsoft Graph for Outlook and Microsoft 365 calendars
CalDAV for Apple Calendar or iCloud-backed calendars

Why this fits

It supports the provider mix you actually described.
It allows strict privacy policies on mirrored events.
It can be deployed and configured through scripts.
It centralizes loop prevention and event mapping.
It is easier to observe and repair than scattered low-code flows.

Suggested behavior

For every source calendar event that passes policy:

Normalize the event.
Decide destination calendars.
Generate a mirror payload.
Create or update a managed event in each destination.
Delete managed destination events when the source disappears or no longer qualifies.

Privacy policy options

Minimal blocker

Subject: Busy
Body: empty
Location: empty
Visibility: private

Medium detail

Subject: original subject
Body: empty
Location: optional
Visibility: private

Full mirror

Subject and body copied
Only appropriate inside trusted domains

Data model

Persist a sync mapping table with:

source account
source calendar id
source event id
source etag or modified timestamp
destination account
destination calendar id
destination event id
sync policy
last synced fingerprint

Operational model

Recommended deployment shape:

containerized worker or small web service
scheduled reconciliation job as baseline
webhook support for Microsoft Graph where available
durable storage for mappings
secrets through environment variables or a vault

Risks

CalDAV recurrence behavior can be fiddly.
Graph subscriptions require lifecycle renewal and error handling.
Cross-provider field mapping must be narrowed to avoid surprises.

Recommended first slice

Implement only:

create/update/delete
timed events
all-day events
private blocker payloads
one recurrence strategy that is tested before broadening scope

Spike outcome

Verdict: preferred target architecture.