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https://old.reddit.com/comments/vj6u54/comment/idhfltv?context=3

From the comment:

This is how I do it:

boot into gparted (make bootable usb https://gparted.org/liveusb.php)

open terminal

run lvdisplay to check the proxmox root and data lv name and path
sudo lvdisplay

resize the root lv (choose size, no curly brackets)
sudo lvreduce --resizefs -L {size}G /dev/pve/root

resize the data lv to use all available space
sudo lvresize -l +100%FREE /dev/pve/data

reboot into proxmox
pray

I do not suggest doing this. Just add another disk.

also: https://forum.proxmox.com/threads/can-i-remove-local-and-local-lvm.122850/#post-534378