Dual boot Mojave-Catalina set up to try out Catalina. Now want to remove Mojave OS

I was running Mojave, but before installing Catalina I set up a dual boot to check that everything worked with Catalina. I'm not a tech expert so I was surprised how easy it was to create a new volume and install Catalina into it. I'm now content that Catalina works for me and runs the apps I was interested in so I now want to make the change completely.

But how do I now move to Catalina? I can find advice online how to set up a dual boot, but not on how, having done so, I can move to a single OS, in this case Catalina. I am looking for advice please

In System Preferences - Startup Disk are now shown two systems: "Catalina macOS 10.15.7" and "Macintosh HD macOS 10.14.6" (which contains the Mojave OS). I suspect simply deleting the "Mojave" Folder on my disk may cause problems, possibly serious ones.

I thought about using Disk Utility to delete both of the "Catalina" and "Catalina Data" volumes so I am left only with "Macintosh HD macOS 10.14.6" (i.e. the Mojave OS). Once deleted I would again install Catalina. Would this work? I want to keep things as simple as possible

If I can work this out then I will probably adopt the same approach for Big Sur in due course.

Hope someone can help.