Create macOS High Sierra and newer bootable recovery partition on a separate standalone disk

What you suggest is no longer possible with Big Sur. When booting to Recovery, you are required to enter a password of an existing account. AFAIK, these passwords are not stored in the Recovery volume. This would make it impossible to boot from a standalone Recovery disk.

Before Big Sur, there is no reason the an AFPS Recovery volume could not be placed on a standalone disk. However, getting the firmware to boot Recovery from such an arrangement might require a third party boot manager such as rEFInd. I an unaware of any manual which would explain how to do this. On the other hand, I can not imagine a reason one would want to do what you are proposing.