The file size discrepancy is usually due to 1000 vs 1024
No, that’s something else entirely. It doesn’t matter what measurement system you use, the drive juggles more sectors than your OS can see.
but filling the drive with random data until its full should wipe the drive.
Only if you assume people can’t access the reserved/unallocated/over-provisioned sectors. If you are only worried about small thieves then this might not be an issue. If you’re handling sensitive data (like medical records for other people or anything with sensitive passwords) then it’s completely inadequate to leave any form of data anywhere on the disk.