Perhaps, the step between 2 and 3 should be go into the 'system>about' menu, keep tapping 'build number' to enable 'developers option', then go back out and into 'setting>developers option' and turn on 'OEM unlocking'. w/o this option turn on, you can do 'fastboot oem unlock'



And maybe perhaps, before doing anything, why not update the system via OTA first? If the OTA zip is OnePlus official, you should be able to flash it using stock recovery, doesn't it?



side note tip: if you wipe the data partition in TWRP, the default would be ext4, but the OP stock use F2FS. I think it may be wise to change that before wiping the data partition.