I hate to be obvious or a broken record, but I'll probably always push WordPress. I've used a lot of different CMS, or frameworks. WordPress has been modernizing. It's gone through a phase where people put it down. Say it's slow. Old school. You name it. But, it's not. It's a PHP CMS.
Since it has been around so long, you'll find examples around how to do anything with it.
With WordPress, I've built blogs, landing pages, odd apps, and full social networks. This year I starting builting a small Applicant Tracking System with it. Because again, it's PHP/HTML/JS/CSS and incredibly easy to extend.
It's only slow if you add the wrong plugins.
It's only not secure, if you add the wrong plugins.
It's only not "modern," if you add the wrong plugins/theme.
And no, I've never worked for them; I don't know anyone who works for them. I've just deeply experienced what it can do.
Search the web for Flarum/WordPress extension. There are a couple, or so. I do wish there were more. It's on my "maybe one day" roadmap to build another Flarum/WordPress extension.
Tip: Using a minimal CSS framework like water.css gets you pretty close to Flarum's look, quickly.