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?

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?