Given the encryption standards for Proton Calendar, I think to push/pull data to other PKM apps would require integration built by Proton vs. what a fan-made plugin could do.

Apps like Obsidian have gotten fairly popular, using standardized formats like markdown for constructing notes and managing data, all while self-hosted and private.

It would be neat to be able to construct tasks for your Calendar time-blocks, link comprehensive notes to recurring events, all while having Proton Calendar provide system-wide notifications without any data being tracked like many other calendar apps.