While I’ve found Big Sur to be one of the smoothest upgrades in recent years, there’s really no major reason to upgrade yet. I generally suggest that, lacking a compelling reason, people wait for a while.
Second, if you have less than 20GB of free space, your computer isn’t able to run at its best. It really is best to keep between 15GB to 20GB free for caches and temporary files. If your computer has a 128GB drive, this can be difficult and it isn’t always easy with 256GB drives either. BobTheFIsherman pointed you to a comprehensive guide. I can distill it to a couple basic ideas: music and video files tend to be the largest files and can quickly fill a drive. You have a decent collection of music, movies, and other video it would be a good idea to invest in an external drive and move those files to it. Data files (word processing and the like) aren’t large and you won’t gain much by deleting them. Except for extensive 3D and RPG games, most programs aren’t very large either.