There is no need to root stock, just to install LineageOS. For that it would have been enough to just unlock the bootloader, no need for Magisk and boot.img patching ...
On the other hand, if having root during use of the phone was your goal, you will find that in your newly flashed LineageOS, Magisk and su are now gone. You'll then have to re-install Magisk and re-do the boot.img-patching procedure (this time using the boot.img from payload.bin, contained in the lineage*.zip you used for sideloading).