Obsidian will change your life

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?

Ghostwriter is another good and simpler one for Linux

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 want a proper journaling app :

Inner Ink

Linked

Rednotebook seems solid

Having markdown means you can use it in a variety of apps but I'm not sure how many have encryption (obsidian and joplin maybe?)

You could also try Trilium Next

I also found Darkwrite. Can export as HTML

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.

If not, what else are you worried about?