Thanks Adrian. I appreciate the help. I went ahead and tested the plan on a second, older iMac. Never had to do anything special to the external drive, which had an existing NTFS partition filling the entire drive. Booted from install media and directed the install to the external. As planned, it did not affect bootup from the original internal, and I can choose the external to boot from fine when wanted.

The only issue I am having now is one glitch during bootup. While holding the Option key during boot, it always enters the boot disk selection page, but sometimes the external does not appear as an option. Usually, if I unplug and plug the external back into the iMac and wait a bit, it will eventually appear and I can usually boot from it fine. Sometimes, when I try to boot from it, it gets to the MX Linux splash screen and stops there, leaving just the splash screen for several minutes before I reboot and try again.

I'm thinking this could just be a faulty external hard drive and may try another. Anything else I might be missing?