cat /etc/pve/storage.cfg gives you the location of your pve storages.
(apt install gdu)
gdu -x /path/to/storage shows you the usage of the storage
(gdu is like ncdu, but much faster)
if you want to have proxmox on your laptop, you should have enough ram, but i see the problem.
(If only i had bought 64GB some time ago ;) )
Practically all my laptops are maxed out in RAM
First, backup your virtual machines and the config in /etc/pve, if still possible.
While a process
(in this case pmxcfs, get the pid with
ps aux |grep pmxcfs |grep -v grep | awk '{print $2}' )
is running you can access the open files via...