When you say “no built-in security”, are you talking about not having https ? Paperless-ngx does have login security with users and passwords. I believe they recommend using nginx as a reverse-proxy server to implement https if you need it.

I like BackupPC, it’ll do what you want but it may be more challenging to learn than some of these other options.

Every time I upgraded the StandardNotes self hosting docker instance, some configuration setting changed and I had to go figure out what had changed and update it. Then they started to require some sort of a key to actually self-host. They were giving out free keys to users who were grandfathered-in but that left a bad taste in my mouth.

Try https://joplinapp.org . You can self-host your database with something like Nextcloud or Apache Server using WebDAV. I moved from StandardNotes after they got tricky about Self-hosting.