Moreover, would it be possible to actually use eight 4TB 2.5” SSDs, or four couples, inside the Pegasus R4i by using those enclosures that merge 2x2.5” inside a 3.5” case in RAID0? Would RAID5-ing four RAID0 couples be a problem? (“double RAID”, kinda like “double NAT”)
combining RAID levels with multiple nodes being arrayed together you typically just merge the levels. RAID 10 is striped (RAID 0) across mirrored nodes (RAID 1). So you're talking about RAID 50 (there's also 60 for RAID 0 over 6). You'll want to do a lot of research about the true impact before deciding to go with a more exotic RAID flavor (I don't consider 10 exotic, 50 and 60 aren't really out there either but you should learn what you're getting into).
For that matter most people don't really learn what 0, 1, 5, and 6 have for impact either.