First, download the proper drivers from dell and run the setup utility (don't manually install from the INF). That will install the full drivers for the changer and the tape drive(s) and stop it reporting "Unknown Medium Changer"https://downloads.dell.com/FOLDER038852 ... 15_ZPE.exe
Second, Veeam already compresses the backup files. The hardware compression of the tape drive can't compress it anymore as it's already compressed so you're only going to get 2.5TB per tape. You can tell Veeam not to use compression but then your backups on disk will take up much more space. There are older posts that suggest Veeam's compression is significant;y better than what the hardware compression on your tape drive can do.