I use proxmox hypervisor to host my VM’s. My provider has ftp server to store backups. Proxmox can’t backup to ftp out of the box, but it’s debian-based and we can use curlftpfs to mount ftp catalog like any network file system. But, in my case, I had issue with ftp share in fstab – sometimes connection broken after inactivity time. Now I use systemd to automount and unmount ftp share and I think it’s simplest and very stable way to use ftp for proxmox backup.