You are right but the main problem is as always "budget".
I know that other solutions would give me more efficiency and safe even if a RAID 6 or 10 too with only 4 drives available does not give more security than RAID 5 but I should try to give the best possible with a 5000/6000 budget.
Two further ways for me, both with some question marks:
1) Using the same solution (a couple of MSGen10) with a Raid1 solution based on 10/12 WDNasWarePro Red disks; the question is: disk so large are supported during install and running ? My experience stops at 4Tb in mirror/R1.
2) More expensive: a couple of Qnaps 6-8 bays (with more raid options) installing Sme inside Qnaps Virtualization Station but I know too few about this (I suppose it's based on KVM/Qemu) and about its performances.
The backup scenario is a 9.2 server (in Esxi 6.5) with about 2,9 Tb data growing in backup up to 5,4 due to usual/physiological file modification and, of course, users devoted to change frequently name to 100 Gb folders
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