Yesterday I helped my friend upgrade his OP6 from eOS-q to eOS-r. It all went smoothly except for a minor stall at the last step. The plan was to restore the phone to it’s previous stock state (OxygenOS 10) described in the XDA link at the top of this thread and then update it to OOS 11 OTA in the phone’s updater. Then install eOS by following the installation instructions, as opposed to the upgrade instructions which miss out the crucial step of sideloading the copy-partitions zip.
Restoring stock with fastboot rom
Host pc was running Fedora therefore a linux shell script was simply created with the commands copied into it; only the fastboot partitions, not persist or the critical partitions. It may be useful to add the line fastboot reboot at the end or at least make a mental note of what the last command was; just so you are certain the process has completed! 
Flashing eOS-r
It can be confusing for anyone who needs installation instructions because there are two sets and both need some corrections. In addition, both sets describe twrp recovery method of wiping but not e-recovery, which is slightly different.
If you’re using e-recovery and need instructions then use the Lineage instructions as a how-to for wiping and format data.
After sideloading the os zip, we returned to recovery home screen and pressed reboot system. Phone started rebooting but went to what looked like part of the Oxygen os recovery, offering options to reset data etc. I can’t remember the exact steps but we selected equivalents to wiping and resetting after which the phone continued to successfully boot eOS-r.
If I was doing it again I would select factory reset in e-recovery after sideloading and before rebooting system.