Actually not supporting port 465 has nothing to do with security. Port 465 is as secure as 587. The difference is about how secure connection is established but both ports will give you the same level of security.
Now, as for Google complaining about less secure apps. What they mean by that is that is your Gmail password is your password to your entire Google account.
Google dislikes their users entering Google passwords anywhere else but login screens run by Google. In this instance, you are entering your Google password in Manager. It’s not that Google doesn’t specifically trust Manager. Google doesn’t trust anyone who is not Google.
As a solution, Google established new authentication scheme which will allow you to send emails through Gmail without exposing your Google password to 3rd party. Manager doesn’t support this and most other applications don’t support it.
SMTP over SSL is secure (port 465 or 587). What is not secure is entering your Google password to Manager. Why? Because your Google password is stored in your accounting file and if you ever send this accounting file to someone else, they could potentially discover your password. If your Google password would be valid only for sending emails, then it is not a big deal. But your Google password can be used to log in into all Google websites which has privacy implications (e.g. someone could see your search history or youtube history)
So for this reason, Google doesn’t like users to do something that might be insecure and this is the case with Manager.
Solution? Create secondary Gmail account with different password which will be used just for sending emails only. Google will still complain about “less secure app” as a general precaution but that’s OK because you know what you are doing.