Startup Security Utility (SSU) - Mac Mini M1 - Big Sur

I'm attempting to allow external disks to boot this Mac Mini, and numerous sources tell me I should see a screen like this when I go into SSU.

However, when I go into SSU, I see a screen like this.

I've tried all the combinations I could find, and I could never bring up the first screen, only the second. And no matter how many times I go to this screen and click OK, I cannot get any external media to boot.

It's already been suggested that I should get brand new fancy drives, but unless I can get explicit permission to boot from external media (first screen), I'm not so confident on spending money on drives I really don't need.

Any ideas how to get external media to boot using the stated configuration?

Much thanks!