i had also this issue with too low memory on update qube.
You cannot increase the system max storage in the update qube (in my case also sys-firewall) because you have to increase the max system storage in the underlying template.
On my system the sys-firewall qube depends on the underlying template debian-11-dvm and this depends on debian-11.
So, increasing the systems max storage size in the very basic template debian-11 was the solution for the update problems in
the update qube.
See the post of my update issue:
https://forum.qubes-os.org/t/dom0-update-failed-on-v4-1-2-r4-1/20058/9