I'm a bit tired from writing all day, so I'm not feeling very wordy, so forgive the terse response. While I'm a big fan of the idea of 'give it a go' via Freeflarum (never tried it myself), I think getting to grips with it yourself on your own platform is also a good idea.
In terms of management via CPanel, I've managed one via Plesk for probably around 2 years now with little to no issues. When I first started using Flarum, the extension manager (https://docs.flarum.org/internal/extension-manager/) did not exist, so it was a lot more tedious jumping into a terminal and managing Flarum. But as things stand at present, unless I'm doing something silly, the extension manager does a good deal of the work.
I did see it was compared to WordPress in terms of management - if set up correctly, it's on par with, and you can do most of what you need to do via the extension manager.
In terms of useful/must-have extensions - I don't know your needs, but MyBB is fairly feature-rich, so I would use the official Flarum extensions list to work out 'what people are using’: https://flarum.org/extensions?tableSortColumn=downloads&tableSortDirection=desc
I would say FOF/Upload ( https://flarum.org/extension/fof/upload) is a requirement for most people (should be shifted into core IMO).
In terms of your actual list:
- https://flarum.org/extension/fof/doorman - handles invites
- Extension manager (comes with Flarum) does everything you'll need - shouldn't need CPanel beyond database setup, although YMMV
- I mean, the license terms you could have checked yourself in a matter of seconds, but sure, here is a page: https://flarum.org/software/benefits that breaks all that down .
- User features : lots of these exist, mostly in the form of extensions and in many cases more than one. In short, do your own research. Because of the way Flarum extensions are posted (with a discussion attached normally), you can do a full look over the history of the extension to see how well it’s supported and in general what issues people are running into.
- Mobile support ... so far as I'm aware, no native apps exist. PWA is an option, but I haven't looked too deep into this.
- Does Flarum meet these requirements out of the box, or will I need significant customization? Depends on what you want to do. Based on what I've read, you could have most of it up within an hour (not including messing with visuals).
- Are there must-have extensions/plugins I should consider? Yep, IMO, fof/upload as noted above, but do your own research.
- How easy is it to manage updates and maintenance via cPanel? Easy, but YMMV depends on how your cPanel is set up and how your knowledge set can deal with whatever issues you may encounter.
- How does Flarum compare to MyBB in terms of performance and flexibility? no idea.
- Any hidden costs or challenges I should be aware of? Depends on what you do, I guess. The core is FOSS, and a massive amount of the extensions are FOSS too. Some cost money (one-off, yearly), others are subscriptions (hate subs), but for the most part, unless you are looking for something very specific, you can coast cost-free. If your needs exceed what's offered, then expect a developer to want to be paid for your request (usually via bounty).
I think you need to dig into the documents, wander the community, and check out the extensions list (all linked above) to find out if Flarum suits you, maybe setup a dev/staging setup and play with the 'product' before making your own decision.
