Flarum is perfectly capable of doing that, but it requires some development. What Flarum allows is setting up your own skeleton, which then boots the whole application. Modifying the skeleton (flarum/flarum) allows for complete customisation and multi tenancy.
Having worked on Laravel Tenancy (https://tenancy.dev) I know a thing or two about this from a technical standpoint, should you be interested in diving into this.