You could very well be right! I'm afraid I don't know, I've not used Proxy Manager. When I switched from nginx to traefik a good few years ago it definitely didn't exist, but I've not kept up with it since.
Yep sounds very reasonable to me. I'd personally recommend traefik over nginx, it's a lot easier to get up and running quickly and integrates natively with Docker. Just slap some labels on your container and boom it's live. Plus, traefik can be configured to easily get letsencrypt certs for your domain(s) automatically, which is really nice.
Weird one I found in the browser was that my password manager auto filling the password field can cause it to spin. I now just copy and paste my password, but add a letter and then remove it so Lemmy knows I entered something. Could be worth a shot?
I had this too. Try shortening your password to less than 40 characters, that fixed it for me.