I figured it out,
The steps I did are as follow:
- use composer to get flarum
- create an empty database
- install a bare flarum, with the same admin/password/email that I used in the old forum
- import the old database to the new one
- try to install manually all the extensions again
- run "php flarum migrate"
- the last step is to just copy over the Assets folder – which I'm still asking from FlarumPro for now
At least the new forum is running very smoothly at the moment.