OK guys, you win. 
I started to see more issues with images not loading again because they were sometimes being shared over http and not https. You’re right, it’s a mess.
I’ve just backed most of the way out of this, leaving the Postgres database in place but back to just one server droplet, no load-balancer and images/Redis stored locally. I’ve left S3 in place for the site backups though. I love how smoothly that works.
I’m back to 15-minute outages with each upgrade I guess, but I’ve spent a total of like five full days on this so far, and I can’t spend any more time on it, so I’m happy that you were all able to put me straight regarding that original tutorial I followed. It’s almost as though they just want people to pay for more Droplets.
Thanks again for your help.