A lot of things here...
You are using 6 monitors, which isn't supported. Use either 3 or 5 (preferred as it would allow 2 mons to fail and still keep quorum).
You have a 3/2 pool set to drive class "hdd", but only have 2 servers with "hdd"...
Scenario
I have a Proxmox VE 8 cluster with 6 nodes, using CEPH as distributed storage. The cluster consists of 48 OSDs, distributed across 4 servers with SSDs and 2 with HDDs.
Monday night, three OSDs reached 100% capacity and crashed...