One of the best tools for database design is the Oracle SQL Developer Data Modeler and it’s free. As a developer who designs databases this tool is indispensable due to the features it has.
The tool can be downloaded from https://www.oracle.com/database/sqldeveloper/technologies/sql-data-modeler/. Note that you need to have an account with Oracle.
The only issue is that Oracle does not provide a .deb file for it, but there is a .rpm file that can be downloaded. As the tool uses Java almost entirely, once the datamodeler-188.8.131.52.1149-1.noarch.rpm file is downloaded, in the file explorer on Linux Mint 21, i.e. thunar, right click on the file name and select Extract Here.
The file contents are extracted to the local system. Open a terminal window and enter the following commands to run the tool:
cd ~/Downloads/opt/datamodeler <enter>
Now you can use the tool.
I had openjdk 11 installed on the system and nothing else needed to be done.