Any reason for not using proxmox backup?
Proxmox
Proxmox VE is a complete, open-source server management platform for enterprise virtualization. It tightly integrates the KVM hypervisor and Linux Containers (LXC), software-defined storage and networking functionality, on a single platform. With the integrated web-based user interface you can manage VMs and containers, high availability for clusters, or the integrated disaster recovery tools with ease.
Any reason for not using proxmox backup? :)
Where should I install it? I guess raspi is not sutable
On the proxmox hosts alongside pve. Then pull each other's backup storage (via VPN/ssh tunnel).
The rpi could pull both storages as a second backup, e.g. via rsync. There is a pve port for the rpi, maybe also for pbs.
(on Proxmox side) why is this better then just doing backups to external hard drive? there is the same functionality but with hassle of running additional service.
Can you give me a bit more details in "The rpi could pull both storages..." part?
Dedup, incremental backups, backup verification
Cronjob with rsync command to copy file contents
Have you looked at Proxmox Backup Server? I have not used it, but I believe it is made to handle remote backups.