Cool, thank you for your feedback!
I try to narrow the gap between our configurations.
Sidenote:
autosnap in Proxmox produce snapshots, which appear on the webGUI and besides of zfs snapshot, it handles other things as proxmox own snapshot does: config snapshot, fsfreeze via qemu-guest-config etc.
This could be a serious difference, as probably using native zfs snapshot never triggers any fsfreeze on the guest, even you use qemu-guest-agent.
At least, as I checked the logs, it seems the issue starts around the time the snapshots happen (cron: 5 * * * *).
Discard also suspicious, but I have less proof about it than autosnap.