What is the use-case for SaunaFS in Proxmox, for image storage like CephFS or do you run VM's on it? Since you are using sata drives, did you run some fio tests or similar to compare the perf to ceph?
If you are using hardware raid, then you format it as lvm-thin by default(i usually do). Then you create Splunk on top of that. But why raid5 instead of raid10, much more IOPS you get from that.
I have one question, did anybody notice a problem with PVE9 and kernel 4.4 VMs? I need to find evidence,but it seems that there is some problem with qemu-guest-agent and 4.4 kernel and pve9 where this machines locks and just gets unresponsive.
Usually Elasticsearch doesnt recommend network drives,because of the latency. But why do you have 6 replicas for some elasticsearch clusters,isnt that too much?