Ivor, I would not recommend backing up different disks with different jobs, since you would have no consistent backup of the entire VM. Also, there might be issues with FLR, since with system disk excluded from the job, you would not be able to restore to original location
Basically, depending on your hardware, backup environment, and backup window, you might be ok with backing up VM of this size. Please review other users' experience
in backing up large VMs.