Logseq
Logseq is a knowledge management and collaboration platform. It focuses on privacy, longevity, and user control. It is Free Libre Open Source Software (AGPL-licensed).
Logseq offers a range of powerful tools for knowledge management, collaboration, PDF annotation, and task management with support for multiple file formats, including Markdown and Org-mode, and various features for organizing and structuring your notes.
Logseq's Whiteboard feature lets you organize your knowledge and ideas using a spatial canvas with shapes, drawings, website embeds, and connectors. You can visually group and link your notes and external media (such as videos and images), enabling visual thinkers to compose, remix, annotate, and connect content from their knowledge base and emerging thoughts in a new way.
In addition to its core features, Logseq has a growing ecosystem of plugins and themes that enable a wide range of workflows and customization options. Mobile apps are also available, providing access to most of the features of the desktop application. Whether you're a student, a professional, or anyone who values a clear and organized approach to managing your ideas and notes, Logseq is an excellent choice for anyone looking to improve their productivity and streamline their workflow.
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It's reasonably doable, I migrated a while ago after logseq decided to destroy my data once again.
I'd say obsidian is not really any more a walled garden than logseq is―the storage is a rather common markdown. If anything, obsidian abuses commonmark less. It's way better as a product, though and it's experience is rock solid.