" Annoying, useless "other" file " is actually the opposite of useless. Without it - the computer would cease to function. Now that Big Sur is on the computer - Apple has hardened the OS even tighter than previous versions of macOS.
In Catalina there were two Volumes that the OS and User saw as One. One Volume was Macintosh HD containing the OS and was Read Only - the other was Macintosh HD -Data containing the User Data.
In Big Sur - the OS - Big Sur is in a Sealed Read Only Volume in an Image Volume that can not be altered, change or removed - period.
Judging from the situation of this computer - so much software or upgrades from previous versions of macOS have been installed. So a lot has been assigned to the " Other " Category - unmanageable by the user as per Apple Statements earlier.
There might be an option but must mention - that option is drastic and will remove everything from the unit - and can not be recovered. It is not the best option but does exist.