Hi,

- I wouldn't use btrfs for your setup - btrfs raid5 is still not stable/production ready and afaik btrfs is tech preview in Proxmox VE
- mdadm is not supported in Proxmox VE - but if this is not a main concern for you and it's working - I would keep it
- so the supported options are:
ZFS .... ZRAID - but better to use zfs mirror(s) for performance (if a RAID controller needs to be used then in passthrough / IT mode ... ZFS needs direct access to the disks)
HW RAID controller + LVM (thin)
HW RAID controller + LVM (thick) + ext4/xfs + qcow2 (for thin provisioning & snapshots)