“Last week, when trying to perform a macOS update on a second operating system on my M1 Mac mini, I only succeeded at the third attempt, after a total of five hours.”
On an M1 Mac, being an admin is no longer enough. The first account becomes the “Owner,” with a key in the Secure Enclave certified by Apple. Boot a second system and it can’t update until the Owner hands over rights. You paid for the Mac, and Apple decides who owns it.