If it still does what you need it to do, then keep it, and use it as is.
There is no workaround to get an update it cannot support if that is what you are aiming at.
2020 iMacs can update to macOS 26 Tahoe which is just below the current macOS 27 Golden Gate and which at this point is still very much supported by Apple with Security patches and vulnerability fixes, and will continue to receive them at least for 2 more years. Also, pretty muchy every App developer will still support Tahoe for app updates at this point and for the forseeable future.
There is no real pressing need to buy a new Mac, speically if your current one still does what you need it to do.
The current release for Tahoe is 26.7.1. Make sure your Mac is updated to that version by going to System Settings ➜ General ➜ Software Updates.
click here ➜ Update macOS on Mac - Apple Support