Backups: Proxmox Backup Server (yes, it can run in a Proxmox VM) is pretty great. You can use something like Duplicati to backup the PBS datastore to B2.
Performance: You can use ZFS in Proxmox, or not. ZFS gets you things like snapshots and raidz, but you will want to make sure you have a good amount of RAM available and that you leave about 20% of available drive space free. This is a good resource on ZFS in Proxmox.
Performance-wise, I have clusters with drives running ZFS and EXT4, and I haven’t really noticed much of a difference. But I’m also running low-powered SFF servers, so I’m not doing anything that requires a lot of heavy duty work.