I had never used Google Messages before, so I decided to install it and see if I could use it without creating an account. When I opened the app for the first time, it actually let me choose to use it without an account right away. I signed in to check if I could switch back to using it without an account, and it was pretty simple.
If you’re already signed in, just click on your profile. There will be a triangle next to your name; click on that. From there, you can either switch to a different account or go back to using it without an account. Give it a try!
However, I would also recommend switching to a different SMS app if you’re not using RCS.