Goodreads is perhaps the best example of enshittification imo. It’s only good now as a way to track your reading lists.
I tried bookwyrm today and it feels quite polished already, like giving you a guided tour of it’s features. Hopefully it takes off as well similar to mastodon and lemmy.
Request entity too large does mean that the data you sent is too big for the server to handle.
I don’t know if you could split in half your book list and import it in two parts, otherwise there is nothing you can do, except post an issue to their issues tracker, probably github.