I just updated to LOS 21, thank you for the instructions on the LOS wiki page.
Boot time is the same or better than version 20, about 25 seconds from cold, so the 3 minute comment is not my experience.
i flashed Magisk by downloading the v.27 .apk and changing the extension to .zip, and then sideloading that with adb. That is the deprecated install method, but I believe @Hook said that is what preserves root after OTA updates, so I did that, no problems, we shall see if root is preserved on the next OTA.