Optimal RAID 6+0 Setup for 40+ 4TB Disks

I am setting up a JBOD containing 44 4TB 7200 RPM SAS HDs. I chose RAID 60 as I prefer drive failure protection over the performance improvements offered by RAID 10. My issue is how to choose the optimal disks per span that would results in a reasonable rebuild time. For example, assuming I leave 4 hot-spares, this results in 40 disks for the following possible RAID setups:

• 2 spans with 20 disks, ~144 TB usable capacity.
• 4 spans with 10 disks, ~128 TB usable capacity.
• 5 spans with 8 disks, ~120 TB usable capacity.
• 8 spans with 5 disks, ~96 TB usable capacity.

I am leaning towards 4 spans of 10 disks as it seems to offer best balance of fault-tolerance (2 of 10 drive failures per span tolerated) and usable capacity (80%, down from 90% for 2 spans of 20 disks).

However, what can I expect rebuild time to be for a single 10 disk span? Web search reveals that even a 10 disk span might not be feasible as rebuild may take too long, thus risks additional drive failure during rebuild. However, many resources on the internet are based on fewer disks or lower capacity disks.

Any thoughts as to what is the optimal setup for this relatively large number of disks?

NOTE: There is backup policy for about 10 TB of data, but not feasible to backup all data. Hence my leaning towards RAID 60 over RAID10. I realize this is not a substitute for backup, but better recovery from drive failure does make system more robust by providing opportunity to rebuild then migrate data to other storage should multiple disk failures occur.

EDIT: Specifications:

• Disks: Seagate 4TB SAS 3.5" HDD 7200 RPM, enterprise grade.
• Controller: ServerRAID M5016 controller, including RAID6 enabled, LSI2208 chipset. See: https://www.broadcom.com/products/storage/raid-on-chip/sas-2208.
• Enclosure: Supermicro 4U storage JBOD 45×3.5 with 2x1400W redundant power modules.
• OS: CentOS Linux release 7.1.1503 (Core).

Thank you for the help.