When I said to use older versions I mistakenly thought you were on a newer version. It should work on 18.04.
I am using python3 and all current versions of ubuntu, ardupilot and all dependencies. As you found earlier, I am successfully exploring all current versions of ROS and Gazebo.
I am using a linux guest machine installed on a virtualbox Windows 11 host.
If you need more help installing the basic structure see this.