How can I script the bootup disk in macOS Catalina without disabling SIP?
The designers macOS pick which commands that are constrained by SIP. To be more precise, certain commands are given the ability to bypass the constrains of SIP. The bless command is not one of the commands that is contained by SIP. To use the command to set the default to boot would require disabling at least part of SIP.
An alternative would be to use a boot manager which can be configured from a macOS script or application. An example of such a boot manager is rEFInd. If installed in a EFI partition, then a password would be required to first mount the partition. If installed to a FAT of ExFAT partition then no password would be required, but would be less secure. The default_selection token can be use to choose the default operating system to boot. Typically, this token and parameters are stored in a file of your choosing. The name of this file is then given as the parameter to the include token stored in the refind.conf file.