You missed the single biggest issue with "New Outlook". It's breaking the law. You went thorough all this to write an article about why you like it and didn't know that?
If you enter email credentials into New Outlook, for ANY email server around the world (on-premise Exchange, Yahoo, Hotmail, Google, etc), it's not that it uses that information to access the email server and being the data to your app.
Nope, instead it grabs the credentials, and downloads your email data to the central Microsoft 365 servers, and your New Outlook is nothing but a portal to view the data at Microsoft 365 servers.
Don't believe me? Setup a on-premise Exchange server behind a firewall, use New Outlook to access it, then Uninstaller New Outlook completely. Now go check your firewall logs, you'll see Redmond and other Microsoft 365 data center IPs constantly accessing your email data, copying it to their servers and keeping it despite you no longer using New Outlook.
Microsoft is breaking every security law in existence by doing this, HIPAA, SARBOX, etc. They are keeping your logins to your email services that are NOT Microsoft 365, and downloading your email data from that point forward, forever, whether you use New Outlook again or not. Changing your password after using New Outlook will show the firewall is no longer having email data requested by the data centers Microsoft owns.
Microsoft won't be punished for this, because they are likely setting up to catalog/cache the entire world's email even if you don't subscribe to Microsoft for email service, and we all know there are government back channels perfectly fine with this since they'll be able to see anyone's email.