Does it help to fully reset a Mac once a year to keep it "healthy"?
Resetting your MacBook Pro to factory settings each year definitely should not be necessary. I do not personally experience any performance problems or functionality problems even though I haven't "reset" in a decade (i.e. I migrate my existing system over when I buy new hardware).

If you do chose to factory reset, which I wouldn't recommend, you would need to sign into iCloud again after reinstalling. If you have your Keychain synced to iCloud, you don't need to start from scratch in amassing passwords and logins. However, you would probably need to login into various stuff once again (like for example login to Adobe subscriptions, Office 365 subscriptions, mail subscriptions, etc.).