erase your Catalina system drive.
Reinstall/restore your older system. You can do that from the backup that you were certain to make (before installing an early beta on your main boot drive)
You should be able to boot/erase/reinstall without the need for a local boot disk - just boot to Internet recovery.
But, it would be a faster method to use a bootable installer for that.
If your TM backup is to a local drive (like a USB drive), you can option key boot to it and get a recovery screen. From there erase the internal drive and restore from a date that still had the previous OS version.