I'm with you 100%. I wish there was an OSS tool that was like Obsidian and cross-platform (no cloud hosting required). Logseq is the closest but the markdown standard isn't fully supported, and they add a lot of custom syntax/metadata.
Probably not what you're looking for, but I'm having a wonderful time running a private instance of Mediawiki. It's only accessible to me in my lan (or through my vpn). It's been my private digital garden for a few years now and i like it very much.
The only thing I truly miss is the possibility of pointing a Wikipedia-like app (might as well be the same codebase of the official wikipedia app, but pointing at my private endpoint).
I'm not sure it has all the features Obsidian has (I haven't tried Obsidian) but it has a fairly large number of extensions: https://www.mediawiki.org/wiki/Category:All_extensions
- Storage of files in a folder that can be seen by the OS to allow sync by something like syncthing
- Moderately good UI (nice to have: live preview of markdown)
- Core features not behind a paywall (e.g. siyuan can't sync, notenook missing important features)
- Nice to have: push notifications for tasks/reminders