Hello Jack-19, I found your document "How to Setup and Use an External SSD as your Startup Disk on a Desktop Mac" extremely helpful and followed nearly to the letter. It was well organized and clearly written. Thanks for the effort you put into this doc. I did run into two issues that you might want to take note of. Not sure if I was doing something wrong or things have changed since the latest update of your doc, but you can read and decide if updates are needed.
The first issue was downloading the Mac OS installer. I wanted to stay with BigSur fo now. Multiple times I tried to download the installer from the links in your doc and by searching directly through Apple Support. They all led to How to get old versions of macOS - Apple Support, but when I chose the link for BigSur, I only got a blank page. Oddly, the link worked fine from my Macbook Air and seems to work OK after my upgrade. Not sure what happened here but any insight that you could provide would be appreciated.
The second issue had to do with Time Machine after I gave the new external drive the same name as the internal drive (Macintosh HD). Time Machine had two drives with the same name to backup and got stuck. I renamed the internal drive and this seemed to fix the problem.
Again, thank you for your User Tip/guidance document. Could not have done this without it!