I’m a big Proxmox fan, having deployed more than 20 systems both at customers and on my home lab this year without drama. However, the Proxmox LVM structure bugs the heck out of me. I prefer to utilize a single LVM Volume due to some weird glitches with LXC images and small root LVM partitions. (Not perfect for production, but works for my environment as everything is stored on NFS)
Run the following as root, on your proxmox server. (Change +200GB to what is appropriate for your system)
lvremove /dev/pve/data lvresize -L +200GB /dev/pve/root resize2fs /dev/mapper/pve-root
The Art Of Awareness 