Yay! I figured it out.
Well, to be honest, web searching did it... this page had the answer and it was simply that Wordpress is a little finicky behind a reverse proxy. So, I can finish the move and downgrade my droplet.
Yay ... so happy. Took me FAR too long to figure it out and tons of distractions ... but I did it.
I feel better about my skills. It took me some time, but I eventually find the answer. And luckily for me, someone else had the same problem. So that's good.
And now it's time to see if I can get a decent night's sleep.
Next up - move misskey and finalise the Nextcloud setup. I really want to try using either Syncthing or Pydio Cells because Nextcloud is maybe a tad overpowered for what I need.