@tizbac recently shared his open source VSS backup client called "proxmoxbackupclient_go" that is currently alpha.
Ideally, Proxmox devs would cooperate with tizbac, since it requires a "Proxmox side patch to add another kind of entry to pxar format with Windows security descriptors in it".
Proxmos devs seem to be using Rust rather than Go, but hopefully they can at least coordinate a common pxar format so that compatible GUI and CLI support can eventually be integrated into Proxmox.
https://forum.proxmox.com/threads/proxmox-backup-client-for-windows-alpha.137547/
https://github.com/tizbac/proxmoxbackupclient_go