Ideas what you can do. These are all SHOULD and not MUST requirements, so pick and choose what you can reasonably do in a realistic timeframe without overburdening yourself. Some of these steps can be outsourced to your community.

You can try to make a twelve factor app but some of their advice is probably not suited for your application. You will end with some 7.5factor app which is fine.

Follow SemVer and provide detailed instructions for upgrading major versions.

Use a build system which is easily installable and a language where you don't have to upgrade dependencies every second for security issues (looking at you, npm/nodejs).

Don't include a webserver which does HTTPS, let the people run their own reverse proxy.

Test your setup with and provide multiple web server configs for nginx, Apache2, Caddy, Traefik.

Test your setup with and provide multiple default configs for bare metal (with a dependency manager), Docker, Podman, Kubernetes, Kata Containers.

If you need a DB, include the possibility to migrate from a self contained one instance SQLite to a multi container pgsql/MySQL setup.

Write database migrations in both directions so people can downgrade on failures.

Make it possible to configure your system via ENV variables, ENV files and config files. Provide instructions on best practices and sane defaults. Explain these defaults and make clear configuration is optional.

Make it possible to disable authentication to add Authelia or LDAP through the webserver. Make clear that this is only to be used for external authentication.

Make it possible to run multiple parallel instances of your software without affecting the database consistency, e.g. for high availability or horizontal scaling.

Provide a versioned, documented API (does not need to be public) and use it yourself for your frontend. Provide a telemetry endpoint which is human readable and machine readable, so Prometheus or a similar system can scrape it.

Making a docker container can make it really painless, for example I’ve found Vaultwarden (self hosted Bitwarden server implementation, https://github.com/dani-garcia/vaultwarden) to be really easy to install. Just docker pull, and what Linux distribution and other particulars about your system don’t matter.

One thing that goes a long way toward making self-hosting easy is to minimise dependencies.

In order of preference (best to worst):

• Your language's standard library.
• Those that are installed by default on most linux distros.
• Those that are available in the main package repos of most distros.
• Those that come from a community package archive. (AUR, PyPI, etc.)

Mind the version numbers, too; try not to depend on library features that aren't widely packaged/deployed yet.

Bonus points for supporting multiple OS, like the various BSD flavours.

Being conservative with dependencies makes it more likely that someone will be willing to install, package, or administer your software. It also helps limit the attack surface, potentially avoiding exploits in the future.

Docker 100%

I think you'll have to learn a bit about security. There is no one article, but entire books written about that... And it really depends on the type of service, the used frameworks and the intended deployment.

I'd have a look at similar software. There are tons of open source projects that handle sensitive information. From files like Nextcloud to contact sync to ticketing and payment information.

Edit: I'd leave Docker as an afterthought, since some people recommend that. It's deployment, not development. And not a means of stopping user data getting leaked or stopping login brute forcing.)

Another thing to keep in mind is resource usage. Software with low RAM and CPU requirements will work well on a great variety of self-hosted server platforms. If your code runs well on an old Raspberry Pi (the original or maybe a Pi 2), it will probably do well in most other environments. This VPS list should give you a picture of low-end platforms that are in use out there.