You could add in support for signing in via OAuth such as a Steam Account, Google Account, or social media stuff like Disqus/Twitter/Facebook.
Essentially a way to sign in via another service and then link the name/avatar and some other basic info to create a minimal user for the itchio service. If they want to be a developer, sell stuff, or post on the forums then they'd need to 'link' to an actual itchio account after creating it. Then they could choose to either login with their itchio credentials or login via the linked 3rd party credentials.
I understand security concerns, but many users enjoy logging in via Steam credentials precisely because Steam natively supports strong & unique 2FA authentication on Steam accounts.