Question about QNAP Ts-453a

Yes, that is possible - but then only Raid 0,1 and JBOD are available. Raid 5 requires at least 3 hard disks. A later extension with additional hard disks is of course possible, but a Raid 0, 1, or JBOD with 2 disks cannot simply be extended to a Raid 5 with 3 or 4 disks without reformatting the existing disks.

If you want a Raid 5 in the longer term, you would either have to invest in 3 or 4 disks from the start - or later export/externalise your Nas completely, delete everything, combine the existing 2 disks with the new 1-2 disks to a Raid 5 (reformat) and play the data from the external backup back onto the new Raid 5 volume... possible, but a bit cumbersome and a certain risk if your external backup kicks the bucket during the copy process.

Sure, no problem. I would then recommend a RAID 1. Attention: If the remaining 2 hard disks are then added, only these 2 can be made into a RAID again. To raid all 4 disks (e.g. RAID 5), the 2 existing disks would have to be repartitioned, i.e. deleted.

Greetings
Schoggi