No practical way for decent sustained speed on host-managed SMR.

CMR zones are only ~100-200GB. After that, every backup rewrite hits random RMW penalty → 3-4MB/s forever.

Only workarounds (still slow):

ZFS with sync=disabled + huge volblocksize (1M) + SLOG on SSD → ~20-30MB/s max

Front with SSD cache (L2ARC or bcache/writeback)

Bottom line: SMR is unusable for Proxmox backups unless you cache it aggressively with SSD. Buy CMR or archive-tier drive instead.