Welcome to Flarum!
Please understand that you've joined at a transition time. We just released Beta 8 (and 8.1), which has been in development for a while and was planned to break extensions. This was done so Flarum could provide a more consistent and robust extension API moving forward. But it means a lot of pain now, as you seem to have discovered.
pstevens Why doesn't Flarum maintain a server for official extensions by users and keep the codebase up to date? A lot of these don't even install anymore.
While Flarum doesn't have an official marketplace, one of our core devs maintains a popular website that does this: Flagrow.io
If you want to see what extensions are compatible with the latest version, you can search Flagrow.io or check out this page to see.
As for updating and maintaining extensions, I urge you to be patient. Some extension developers (including myself) simply have not had the time or energy yet to update their extensions. Some are in the process of updating them (and often are providing progress statements on their extension discussions) . And some have been taken up by a group called Friends of Flarum who intend to maintain old extensions abandoned by their creators.
I hope this helps you understand and have a better experience. Welcome to Flarum!