Upgrading from Mojave to Catalina, but not the newest version.

I am trying to upgrade my mac mini from Mojave 10.14.6 to Catalina 10.15.5. I run a version of Protools that is not supported by any OS past 15.5 so I am needing to avoid the final Catalina 15.7. I can download 15.5 from apple support but in order to run it I need to have Catalina already installed. The only installer apple currently offers is for 15.7, but I know that once I install to 15.7 I won't be able to downgrade back to 15.5. Can anyone offer any advice here, or can you point me to a Catalina installer for any version between 10.15.0 to 10.15.5?

Thanks.