Storage

Please Read this from Apple Regarding Upgrade to Big Sur. Special attention on the compatible computers that qualify for the UpGrade and second, the Empty Space Requirements for a successful Big Sur installation.

Strongly suggest have a Time Machine Backup Before attempting the upgrade, possibly 2 External Tested Bootable Clones each to separate drives and remember since Catalina 10.15 and also Big Sur, 32 Bit Applications / extensions are NOT Supported so need to take inventory before going forwards. Lastly, if there are any AntiVirus, Disk Cleaners, Optimizer etc - remove them not. They create havoc and really do very little and can corrupt the macOS

It sounds to me like it is time for you to purchase an external disk for storage, so you can move files to it from your computer. Now days these drives are very inexpensive, easy to install with a USB cable, they are very compact and most do not require any external power. You may have to reformat it, though. Then, problem solved.

As @ Ronasara has suggest and a very good idea too - get an external USB drive and move fills of the Internal

Very welcome and with due diligence it should work out.

Very Best in the New Year