I don't have a Pro1x, but I believe the process of updating Lineage is the same as for the Pro1 and I will answer your questions from that assumption.
Your steps 1-5 are correct except that, after you do the adb sideload, you need to reboot to recovery. This is because the update (which is actually the full installation with the updates) is installed to the inactive slot. Rebooting to recovery makes the inactive slot active. Now when you do step 5, you will be actually booting up the update.
A. There is no formal backup for either the Pro1 or Pro1x. No TWRP. You can backup your