Yeah you are correct I posted before realizing what sub I was in. I would kill for a opensource version I have tried all of them but keep coming back to obsidian
You should at least play around with the export options and do regular exports. You really don't want your diary of over 5 years to be stuck in an unmaintained app. And since this seems to be supporting exporting into markdown - maybe try an export and checkout logseq or Obsidian?
QOwnNotes is a fast loading, Qt based, plaintext editor that supports markdown and preview, plenty of helpful plugins, heavily customizable, has nextcloud support, supports workspaces in the form of note folders.
No affiliation but a happy user and also sponsor it.
If you are afraid someone gets access to your computer and then has a ready to use backdoor in that app and you can lose valuable prose, then use something different.