It's not a simple task, so I won't list many specifics, but more general principles.

First, some specifics:

• disable remote root login via ssh.
• disable password login, and only permit ssh keys.
• run fail2ban to lock people out automatically.

Generally:

• only expose things you must expose. It's better to do things right and secure than easy. Exposing a webservice requires you to expose port 443 (https). Basically everything else is optional.
• enable every security system that you don't have reason to disable. Selinux giving you problems? Don't turn it off, learn how to write rules to let your application do the specific things it needs. Only make firewall exceptions where needed, rather than disabling the firewall.
• give system users the minimum access they require to function.
• set folder permissions as restrictively as possible. FACLs will help, because it lets you be much more nuanced.
• automatic updates. If you have to remember to do it, it won't happen. Failure to automate updates means your software is out of date.
• consider setting up a dedicated authentication setup like authellia or keycloak. Applications tend to, frankly, suck at security. It's not what they're making so it's not as good as a dedicated security service. There are other follow on benefits.
• if it supports two factor, enable it.

You mentioned using cloud flare, which is good. You might also consider configuring your firewall to disallow outbound connections to your local network. That way if your server gets owned, they can't poke other things on your network.