I picked up obsidian because it is a perfect diary app w/ templates and daily notes built in. But it's so damn customizable that my obsidian notebook has become an all consuming passion of knowledge base and personal project managment that requires me to be productive IRL to generate more content for me to catalogue. Really appeals to the data hoarder in me, been a game changer. Highly recommend. Perfect 5/7.

Obsidian.rocks

Obsidian can be almost anything you want it to be. Try searching out some videos from folks who use Obsidian for journalling.

A note taking app can be turned into a diary app if you only create notes for each day.
Even better if you want to then expand a section of a diary entry without actually modifying it nor jumping between apps.

Obsidian can easily help you tag and link each note and theme/topic in each of them.
There are several plugins for creating daily notes which will be your diary entries.
Also it's local only, you can pair it with any sync service, the obsidian provided one, git, any cloud storage, or ones which work directly with the files like syncthing.

Just curious, what are the special features you expect from a diary service/app which a note taking one doesn't have?