I think the Fusion 12.x licence key can be entered into the 11.x installation. Vaguely recall reading in one of the posts in this communities forum.
You can download 11.5.7 from this link
https://customerconnect.vmware.com/en/downloads/details?downloadGroup=FUS-1157&productId=798&rPId=55...
Or this
https://customerconnect.vmware.com/en/downloads/info/slug/desktop_end_user_computing/vmware_fusion/1...
