I’m answering my own question to close up this topic.
So the thing that worked in the create an account screen was leaving the first password field blank… adding any kind of username and then using the YunoHost instance password as the password for the account.
I managed to get it running for a day… but I turned on 2FA and the next time I had to log in, I got an error message when entering the 2FA code. Ditto for recovery codes, so I finally gave up on Sharkey 