Backup or Expand

[Enquiries]

I purchased a Qnap TVS-H874 on your recommendation, populated it with 8 new 18TB EXOS drives in a RAID 5, 2 1TB Crucial NVME, added the 2x25GB NIC and upgraded the RAM to 64GB. I also put the same NIC in my PC for better speed/compatibility. I upgraded from a Terramaster F5-422 that I ended up sending in for warranty replacement and have 5 of the same 18tb EXOS drives left over which I would like to utilize. I am sorely tempted to purchase 3 more drives and use the 8 drives total to populate a Qnap TL-D800C expansion and have much more space, but the logical part of me says I should do a backup NAS instead. I am thinking about the TS-873A and I could use the leftover 32GB of RAM to upgrade to QUTS Hero.
Two questions:
1) From my understanding the TL-D800C can be used to add another RAID 5 essentially resulting in a RAID 50. Does the 10Gbps of USB gen 2 throughput on the TL-D800C add to the throughput I am currently getting? (AJS Tests at over 2000MBps)
2) Would it be smarter to backup

[ed]

Qnap will not allow RAID50 across the expansion unit. It will need to be separate RAID5. But you can set up backup tasks to copy data automatically.
USB will be much faster compared to LAN-based backup.
Having a separate NAS would allow you to place it in a garage, basement etc. that can help with flood, fire, theft etc.