If you’re already going to the trouble of setting up ZFS for the two NVMe disks, I would suggest setting up a separate pool on the HDD as well. It will save you from monitoring two different filesystem types and give all the ZFS features, checksumming, compression, snapshots, etc… Do make sure your server has a decent chunk of memory through, as your VMs will be fighting the ARC for ram…

A proxmox root device uses barely any space. Mine usually sit around 12-14 Gb used. Writes are also negligible. DRAM less ssds are not a problem. I would suggest installing Debian first, so you can properly partition your root device before installing proxmox (https://pve.proxmox.com/wiki/Install_Proxmox_VE_on_Debian_12_Bookworm). You’ll have much more control over your disks and networking than using proxmox’s own installer. Start with a 64Gb root partition, and leave the rest of the drive empty for future use/SLC cache.

Unless your VMs are somehow high volume data writers, like a proof-of-space-coin, I wouldn’t worry about it. Homelab setups rarely reach anywhere near the kind of write endurance of ssd’s.

Your VMs are not going to write to the root device, so it won’t matter.

You won’t notice the difference in performance of a filesystem on a rotating harddisk. Look for other useful features, like at-rest-encryption, and checksumming for bitrot protection.

I would use a filesystem with checksumming, rather than relying on any point-in-time check to monitor HDD’s. Assume they will all fail eventually, because they will.

Security and bugfixes, after one or two rounds of testing by early adopters/key users. Preferably through some form of automatic updates.

New features and breaking changes, or anything that requires the end-user to pay attention, I’d say no more than 4 times a year, and using a non-automatic form of update. The hard thing is getting the user’s attention on the changes, and not just clicking next and then having a broken or insecure installation.