Can't rebuild the volume on ReadyNAS NV+ v2

Your NAS is 7-10 years old.  It was replaced by units runing OS6 about 7 years ago.  Perhaps you bought it used and don't realize that.  But blaming Netgear for failure to support drives of a size that was not available when the NAS was manufactured is unreasonable.  It is also a home NAS, not a business NAS, and few at that time needed huge amounts of space for home use.  The max drive space in the spec is 12TB, which is based on a RAID5 of 4x4TB.  There is only one 4TB drive on the compatibility list, though others certainly do work.  It may be possible to push the space beyond 12TB once a valid volume is created, but I wouldn't count on it.

Yes, you probably do need a new NAS if you need that kind of storage space.  If you purchased the NV+V2 new, then it has served you well and it's time to upgrade.  If you purchsed t used, you have learned a valuable lesson to do your homework regarding the capabilities and history of a product before you purchase.

If a new NAS is not a possibility, see if it can create a volume on just two of the 6TB drives.  If it does, try expanding that or creating a second volume (which would require you switch the unit to FlexRAID mode instead of XRAID).