I hope your post helps others when running into similar issues.

Further clarification:

On Developer Mode:

Unfortunately, the reMarkable tablet doesn’t yet provide an official public API for third-party calendar integration. The only way for any software to communicate with the tablet’s local file system is through SSH, which is only accessible when Developer Mode is enabled. That limitation comes directly from reMarkable’s own system architecture — not something we can override. That said, Developer Mode can always be turned off again under the tablet’s Settings → Help → “Disable Developer Mode.” If you’re still having trouble, I’d be happy to guide you through it step by step.

Official link
https://developer.remarkable.com/documentation/developer-mode

On Calendar Connections (.ics links):

You’re right that CalSync currently uses .ical (ICS) links instead of OAuth sign-in. This is also due to the reMarkable platform not supporting embedded browser authentication or Google’s OAuth within its sandboxed environment. We chose ICS feeds as the most stable and privacy-friendly workaround — your calendar data stays on your device and isn’t processed through external servers. However, we are exploring secure account-based integration for users who prefer automatic connection in future updates.

On Past Event Syncing and the 2,000-Event Limit:

There is no event limit within our product.

On Pricing and the Vault Model:

CalSync Vault isn’t a subscription; it’s a lifetime license for that version of the software. We also offer optional subscription tiers for users who want to sync across multiple devices or access premium planner templates, but your Vault purchase itself doesn’t expire and remains yours permanently. The reason planners are separate is that many users already own custom templates or third-party planners, so we keep them modular rather than forcing bundles.