My installation process on nginx is usually to simply keep whatever is default and change it according to Flarum install documentation. As long as the default install is made based on a sane guide or made automatically by your host, it should be fine.
More specifically in my case, I use managed VPS configured with Laravel Forge, then just alter the web root and link the Flarum nginx file. And voilà.
HTTPS is configured automatically via Forge so I don't have to take care of that, but otherwise plenty of guides will explain how to configure it, including the one from Let's Encrypt themselves if you want to use their certificates.
User39 to answer your question I'd say most default configuration should be fine to build upon. If you have more specific questions, please let us know.
Have you managed to fix your initial 404 error ? It seems like you had removed or not added the necessary PHP file handler to the config. How this part of the config needs to look like depends on how you installed nginx and php, but usually you can just copy from an example host config.
