Debian Buster requires the option x-systemd.automount
in fstab to mount SMB shares. So my fstab line looks like this:
//<SERVER_IP>/video /home/shares/video cifs vers=3.0,rw,user,credentials=/home/<USER>/<CREDSFILE>,uid=<USER>,gid=<USER>,x-systemd.automount,_netdev 0 0
Magically, this mounts the share when you first access it rather than from boot time.