How Do I Restart My Mac Running Catalina using USB Drive Running Mojave?

Can you help me to create a Macro which will restart from my Catalina System (MACINTOSH HD) into my Mojave System (attached USB Backup disk)?

I'm fairly certain there is no way to do this.

Years ago there was an app called QuickBoot but it ceased to work with SIP.

I've looked around for another tool, but there isn't one, as far as I can tell.

Would love to be wrong, of course.

Have a look at rEFInd (said here with no experience using it).

Notably, there's a page on using the app on 10.11 and later systems which include System Integrity Protection. Skimming, it sounds feasible, if convoluted, as things are when working round SIP.