TB54 hello and welcome!
but I don't really understand if the extensions enabled and their options are included in the database you can save from freeflarum settings, and if it's the case the only thing left would be the CSS additions to copy, which is nothing
Database dump downloaded from FreeFlarum (or technically dumped directly from Flarum) contains everything (including enabled extensions and custom CSS from Appearance settings). Everything except for images. You can download avatars at the moment along with the dump at FreeFlarum, but in the future you'll be able to download other extension assets too.
Is there any advantage in installing flarum on your own ftp?
Yes, the rest of the advantages that you have listed, plus: not being tied to FreeFlarum's limits (for example, user limit - although not enforced at the moment, we may ask you to move away if your forum is way too big for FF to handle).
I have started a test forum on freeflarum to see what was possible on flarum in terms of appearance/customization, in the idea to later copy it on a "real" flarum forum on my ftp.
Congratulations on that! That's actually what's the goal of FreeFlarum - to provide you a starting point for your community to migrate elsewhere later. FreeFlarum doesn't assume that you will be using a free service to host a production forum consisting of +250 users (although some people do that, and then there's a problem if something goes wrong behind the scenes which will make all forums temporarily offline - self-hosting would negate that).
If you are not experienced with Unix/Linux or don't know how to install Flarum and migrate, but you have a server with SSH access, you can message me at Discord (Kevo#7590) and I will migrate your forum, if I have the time to do so (I have already successfully migrated at around 3 forums and don't ask for anything in return, however supporting me is always very welcome).
