I just found this after searching for the timeout error - Yahoo mail requires that you generate a 3rd party app password for Gmail to access your Yahoo mail.
Follow this link: https://help.yahoo.com/kb/access-yahoo-mail-third-party-apps-sln15241.html
Personally I consider the app password requirement to be a red flag.
App passwords are a workaround for insecure and obsolete apps which couldn't handle OAUTH or the other modern auth frameworks when 2FA became mandatory.
But it's also a signal that the app publisher (Google) is fundamentally not committed to interoperating with the target service (Yahoo!)
You can keep trying to shove a square peg into a round hole here, or you could opt for a smoother experience by using a compatible and supported app. Doesn't Yahoo! Mail have its own native interface? Either mobile or on the web, that's where you'll enjoy the most features and functionality. Your software will also be fully supported by the same company.
It's very tempting to consolidate all your mailboxes into one app. I understand for sure. But you'll experience significant inconvenience over time, and you'll eventually question whether it was worth it.
