Rust already has one of the best text editors: helix.
Then just take notes in markdown.
You can just use the filesystem and markdown files. That is a common way for applications to store note type data locally. This is what tools like Obsidian do and has the advantage of being easy to get the notes out if you want to use a different tool.
Note that Obsidian parses all the files and uses the browser's built in database to store indexed metadata and links between note files. It's how they have a responsive search function. You could either use sqlite, ad-hoc your way with flat-files.
Of course, you could parse, walk links, and search through all files on the fly all the time. Text files at personal-notes scale should cache really well, and it might still be faster without the browser-tech overhead, but it's a toss-up
And just use a filesystem that supports snapshot and bam! You've got version control too.
Is it a fully local backend? If so, you don't need a database. JSON could be enough.