As far as hardlinking goes, I think this is your answer (as I am not a guru on that bit, or anything really!)
https://forums.koozali.org/index.php?topic=49341.0Effectively files are deduplicated to save space.
The only way you are going to realistically do this as far as I can see is:
1. Remove some of your backups to reduce space
2. Create a LVM with multiple disks to create one volume but no redundancy
3. Get more/bigger storage - possibly run a RAID array that you can enlarge by adding disks?
I'd be tempted by a RAID array so you are not dependent on one huge disk - if that fails what is your strategy at that point?? Several smaller ones in an array are always more fault tolerant.
I have my main affa backup on a RAID 1 array but it is tiny compared to yours. If I was to increase capacity I'd get myself a storage server with several disks and create a decent array - Raid 6 or better. It will have some inbuilt protection for you. If you really have that much data to protect then you really need to start getting spendy to protect it properly.
Note you do not need to run LVM if you use a Hardware RAID card. Just format as say ext4 and you can then add drives and expand the 'drive' as required.