If you trash files in root, it will go to root trash and will need root privileges to delete them.
If you trash files in home, it goes to the home trash directory and root privileges are not needed.
You may need to clear out journal log entries, clean apt cache, run autoremove and if you have Timeshift or DejaDup, remove old backups. You only need one or two backup copies...
But those backup programs often just keep saving new copies, stacking them up in massive bricks.
Clear log entries:
journalctl --vacuum-size=500M
Clean apt cache and automatically installed dependencies that are no longer needed:
sudo apt clean && sudo apt autoremove