Late to the party, but being lazy (and traumatized from the camera breaking before) I take a more relaxed approach to installing updates.
Can at least confirm dirty flashing worked fine. From a slightly older LineageOS 21 build (November or October I think it was). Everything seems to be in order, GApps + Magisk too.
Had to use fastboot to flash the patched boot image myself. This tripped me up for a few minutes, after not having done it in a while. Are you sure you used adb sideload there?