Hi,
Except for verification, that it always has been a pain unless you control de CA and the peers with certificate that connect to you, and so I haven't set them as you, it works for me.
Also I force TLS 1.2, but it is unnecessary as Google already negotiates 1.2 if you remove that from the config file.
Maybe your problem is in the security changes you did.
Unless they explicitly request it, as Yahoo/AOL is doing lately (change on October 20th) I wouldn't touch anything in the account.
And even then, if you do, the app-password is for your mail client, not for stunnel. Stunnel just provides the secure channel, not the authentication method.
Said that, check stunnel log, or post it here, because the main question is, does stunnel connect without errors? If so, nothing else can be done, regarding Stunnel.
It is impossible to contact Google, but you'll have to find a way to figure out what is going on with your account or your mail client.
Regards.