When expected files is not found - it could be file system error that caused file to disappear - it does not have to be - that is why I suggest to check the filesystem for error.
The journal has also entries that indicates errors with the selected theme.
A second suggestion is to switch to the default Plasma Global theme.
There is no need to use old drivers (<590xx) as Nvidia 30xx is supported by the mhwd video-nvidia driver.
As for booting a live ISO and doing maintenance - file system checking - see [root tip] [recovery] Basic Manjaro Linux Rescue and Recovery.
While file system maintenance is not specifically mentioned - the requirement - a live environment is described.
When you have started the system from the live ISO, locate your system disk
lsblk -f
Then - depending on your chosen file system - e.g. for ext4 on /dev/nvme0n1p2
sudo fsck.ext4 -p -v /dev/nvme0n1p2