We have successfully upgraded to 0.18.2. It only took the best part of 5 hours, 4 restored backups and a whole temporary dev site spun up to figure out it was one tiny setting in Cloudflare that was causing all the issues. The joy.
I'm off to bed with an incredible headache. I sincerely apologise for all the downtime this evening/day, it is not something I want to make a habit of.
For anyone still having trouble posting, you may need to log out and back in again. Worked for me